Your question is: What do they gain?
They count and report the time knocking on doors and the literature they place or leave at the doors. This is the same with the time spent with you in studying JW religion. The hours reported show how 'spiritual' the JW is.
Unfortunately, there are not many perks for a JW woman so about the most she can qualify as is called a 'Pioneer' which means she will have a quota of hours to spend in preaching each month. Most JWs are not Pioneers but are called Publishers. Publishers do not have a quota but are expected to report at least the national average of hours preaching (usually 10 hours/month).
If a baptised male JW consistently attains the national average, he may then qualify for additional congregation 'privileges' (prestige of position) as a Ministerial Servant and then Elder.
A 'study' (ie: you) who becomes baptised, allows some additional bragging rights for the JW who studied with you and maybe considered to be an indication of God's blessing on them.