Why are there 3 programs in s6 that do the same thing?

[joaog.bastos]

Hi.

s6 has three programs to set UID / GID of a process: s6-setuidgid, 
s6-applyuidgid and s6-envuidgid. All three of those seem to do the same thing, 
with some *slight* differences. Why are all three included in s6?