
The previous reviewers have stated the benefit(s) of this card: providing versatility and the ability to make changes in your deck later in a campaign. A good example someone gave is with Sleight of Hand. Until you spend the xp to get Lupara, you don't need Sleight of Hand yet.
Correct me if I'm wrong, but Adaptable also lets you swap out the out-of-faction cards in your deck too!
For example, the Dunwich Legacy cycle investigators (Jenny the Rogue) are allowed up to 5 level 0 cards outside their faction. With Adaptable, you can choose those 5 colors to serve a particular purpose (maybe, make Jenny into a decent clue gatherer with Seeker cards). Then, once you have the xp for the amazing Rogue clue-gathering cards (Lola Santiago, Lockpicks), then you can substitute those Seeker cards for different cards (maybe something to make her more able to handle enemies so she can investigate locations alone).
TLDR: Adaptable lets you swap around level 0 cards, across ALL factions that investigator is permitted to include in their deck. Versatility, from being "Adaptable", can improve efficiency and survivability.