A few things to consider......
Cropping in post gives you the opportunity to position the subject for the strongest composition. Many photographers tend to center the subject in the frame when shooting. But if the subject is looking sideways, it is better to leave more room on the side to which it is looking. Images with horizons are much stronger if the horizon is either very high in the frame - or very low in the frame.
Cropping in post may give you the option to crop OUT distracting elements in the image.
AND 1x1 square crop is often not the best crop for an image. By cropping in camera you cannot later decide to do a pano 16x9 crop for expansive landscapes, a 4x3 crop for a large 24x16 print, etc.
If the horizonal is not level, you lose even more of the image when you straighten the horizon.