[papln]

One reason:

This isn't TS-specific; it's common in Python too: coercing to Bool has language semantics (for whatever language you are in) which often don't match the application semantics of your program. Application programmers don't (and shouldn't have to, but for the language's over-eager coercions) always think about the boolean semantics of all their objects. In particular, None and empty/zero object are both False in Python, and Python style/linters push you to avoid explicit comparison to None, which gets weird when your application wants to treat empty objects as True because they have differen semantics from None. (For example, in a security function, None may mean no-op / fallback to default, but Empty might mean "Reject all".

[WorldMaker]

Another reason that is TS-specific is that of JS functions can be highly variadic in the number of arguments and a lot of subtle runtime bugs can be found in blindly passing arguments without checking their count. If filter changes from returning only one thing to say two (for instance, an index count), Boolean may produce a runtime error for having too many arguments, may silently ignore extra arguments, may interpret an extra argument as changing the behavior, or some combination of all three depending on strictness versus compatibility level, executing browser, phase of the moon, etc.

Boolean itself I've not had trouble with, but things like map(parseInt) is the big one that bites a lot of junior developers all the time in TS/JS. (parseInt takes an optional second argument for radix, so in cases where the second argument returned by map is an index count, which is likely, you get it parsing in base-0, base-1, base-2, … which is almost never something you'd do intentionally.)