Many people like designing their own Edges, Hindrances and powers, however the standard suggestion from other savages is to apply trappings to an existing ability when possible, rather than invent a new one. Even the "New Edges & Hindrances" section in the GM chapter of Savage Worlds Deluxe recommends this approach, and in general it's a good guideline to follow.
But there's a flip side of the coin; trappings provide flavor, and many Edges, Hindrances and powers already include a baked-in trapping designed to give a default flavor. This is why it's important to create appropriate abilities when you're designing your own setting - not just by giving them cool mechanics, but also by connecting them thematically to the setting.
If you're running a private game you can often handle the trappings yourself, without needing to explicitly invent any new abilities. But if you're designing a setting for others to use, I believe it is worthwhile creating some appropriate abilities with their own baked-in trappings.
Disclaimer: Of course, trappings can be applied to pretty much anything. You can apply them to skills (e.g., Lockpicking as a "knock" spell), equipment (e.g., treating a bow as a wizard's staff that fires energy bolts), monsters (taking the statblock for a vaguely similar monster and adjusting it on the fly), and so on. However in this post I'm focusing on Hindrances, Edges and powers.
Even if you just look at the core rules, you'll see many Hindrances that only differ in terms of trappings. Outsider, Ugly and Mean all give -2 Charisma; Greedy, Vengeful and Vow all reflect a personal driving goal (with the level of risk determining whether it's a Minor or Major Hindrance); Arrogant, Cautious, Curious and Quirk all represent a distinctive behavior exhibited by the character. These 10 Hindrances could just as easily have been reduced to 3 Hindrances, but the baked-in trappings provide inspiration and useful examples for players.
You'll see the same thing when you look at specific settings. 50 Fathoms has a Branded Hindrance, Lankhmar and Rippers have Jingoistic, Weird Wars Rome has Foreigner, Hellfrost has Black Sheep, Guild of Shadows has Grubby Urchin, Drakonheim has Deathly Visage, and so on. In theory all of those Hindrances could have been treated as trappings for Outsider (which itself could be merged with Ugly and Mean). But turning them into new Hindrances and applying a thematic trapping with appropriate descriptive text helps evoke the flavor that the setting is trying to capture, inspiring the players with ideas for character concepts that better fit the setting.
The same principle applies to Edges, although in this case you need to be particularly careful about stacking bonuses. Attractive and Charismatic could just as easily have been treated as trappings for the same Edge, but by splitting them into separate Edges, players have the option of taking both and adding their bonuses together.
Some settings work around this issue by removing certain Edges. For example the Make it Work Edge in Frost and Fang might initially appear to be a weakened version of McGyver with setting-specific trappings, however the Accursed setting has removed the McGyver Edge, and without that option, Make it Work becomes much more appealing. Another workaround is the "no stacking" feature of Professional Edges, which I used when designing the Stealth-based Professional Edges for Guild of Shadows.
A common complaint from newcomers to Savage Worlds is that the powers are generic, bland and boring. This is almost always based on a misunderstanding of the role and importance of trappings - the powers aren't really intended to be used out of the box, but as templates for designing custom spells. It was this misunderstanding that I attempted to address a few years ago with my Savage Spellbook fan supplement.
Savage Worlds describes trappings as "the heart and soul of the powers system". However much like Edges and Hindrances, many of the powers already have baked-in trappings, and this can often lead to confusion. You'll see some people arguing furiously that the zombie power should only be available to evil spellcasters because it's "necromantic", while in the next breath explaining that bolt is only evil when you apply a necromantic trapping to it. Of course zombie is really just a summoning spell with a baked-in trapping, and trappings can be removed just as easily as they can be added.
But if you strip away all the baked-in trappings from Savage Worlds, the risk is that you can end up with a small number of completely flavorless powers - when I tried this in Savage Abilities I was able to reduce the total number of powers down to just five, from which practically any other power could be created by applying the appropriate modifiers and trappings. This is useful from a design perspective, but not a good way to sell the magic system to players who are already complaining about the powers being too bland.
By contrast, spells in D&D always have baked-in trappings, and there is no concept of changing those trappings (other than in a very limited fashion through Metamagic Feats). This is what leads to huge spell lists, something that many savages hate, although I've experimented with this approach as well.
Overall, most abilities in Savage Worlds seem to strike a balance between flavor and flexibility. Many have baked-in trappings, and this seems to be particularly important for setting-specific Edges and Hindrances, which should help promote and evoke the flavor of the setting.
However there is generally the expectation that abilities can also be customized with trappings, and tailored to the individual player.