You need a record of your T/Offs and Landings so you can easily check,
or prove if you have to, your currency (3 x T/O and 3 x Landing in previous
90 days) to legally carry passengers.
In Multi-Crew both PIC and P2 have to be current.
I don't believe it matters if you were PIC, PICUS or P2 - what does matter
is were you "Pilot Flying"? You Log all T/Offs and Landings when you were
"PF".
It is a good idea for a Student to record, not only the number of T/Offs
and Landings, but also what sort ie: Glide, Flapless, Short Field, X-Wind,
T&G or FS.
Because of above, it is also good practise for Instructor to also record the
same details.