In attack on Titan, what's the point of the shifter Titans becoming soldiers? In attack on Titan, what's the point of the shifter Titans becoming soldiers?
For the most part, it's for stealth and infiltration - if Annie, Reiner and Bertholt had walked into Shiganshina and asked to join the survey corp in Titan form, the first arc would probably have gone a lot differently! 

As their main objective was to find and capture the 'coordinate', they had to maintain a low profile and draw as little attention to themselves as possible. Gathering information on the situation inside the walls was made much easier by remaining human.