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.
Pretty heavy spoilers below:
In 845, Marcel Galliard, Bertholdt Hoover, Reiner Braun, and Annie Leonheart depart from the mainland and initiate Paradis Island Operation. The goal of the mission was to infiltrate Wall Maria, Rose and Sina in an effort to recover the Founding Titan. However, while they knew the individual in possession of the Founding Titan was a member of the royal family, they were unsure of the exact identity. Thus, this operation relied heavily on the infiltration of the military ranks in order to gather intel and ascertain the identity of the Founding Titan. For this reason, Annie, Reiner, and Bertholdt each secured a position in the top ten recruits of their training class, which allowed them to freely choose which division they would go to. The original plan was to infiltrate the ranks of the Military Police where it would be easier for them to determine which of the royal family members was the Founding Titan, but the plan is then revised when Eren is discovered to be the Attack Titan, and Reiner and Bertholdt join the Survey Corps (the division Eren chooses) in order to gather more information on the titan-shifter.