The modified engine someone else is selling could have a potentially important extra features.
For example a company might try to sell a version of engine which has been ported to a console which original engine doesn't support. Game porting companies are very common and if it's their main business then they will usually have inhouse libraries or modified engine versions which significantly simplify the porting process.
That's exactly what's happening with open source game engines like Godot. Their documentation lists almost a dozen companies providing porting service for godot games. That isn't necessarily a bad thing, but it's up to author of game engine whether they want to allow others to profit from their work in such way.
Seems like currently Defold supported platforms cover most of the popular consoles, it was probably not the case during early development of engine when license was chosen or in a few years when next generation of consoles come out. Someone might also be selling a better console support than what defold provides out of the box. Beside the consoles there is also stuff like integration with various PC stores like GOG,Epic and others. Its not necessarily a huge work, but plenty of smaller devs want to focus only on the gameplay aspects. So once a game is finished (and you are tired from development process), buying anything which significantly reduces porting/integration effort can be an easy choice.
One more example of major feature which can require tight engine integration and motivate buying a modified version of "free engine" is multiplayer support. Good multiplayer support can be quite tricky with some game genres being harder than others. There have been many attempts at providing magic multiplayer solutions which under the hood automatically synchronizes all game entities without developer thinking about it. Such approach isn't necessarily going to be as good playing experience as designing the game with multiplayer support in mind from day 0, carefully thinking how the game state is organized, what when and how is synchronized. But that requires planning ahead, technical expertise and suitable budget. Commercial multiplayer middleware for existing engines are also not uncommon.
Whether something like that is considered an addon or modified engine version depends on exact licensing terms and the exact implementation details how game engine and addon code is organized.
A slightly different example - game engine built on top of game engine is RPG maker. For a long time RPGMaker has been it's own game engine. But few years ago developers of RPGMaker made a version of RPGMaker which is built on top of Unity. Plenty of other genre specific engines (especially for fighting games) built on top of general purpose game engines.
Again the line between modified engine, addon and game with builtin editor is tricky.