It helps to practice with others.

Consider a traditional analogy:

A man leaves his village on horseback with a definite destination in mind. He rides furiously through rough terrain. On his back is a leather bag of sharp volcanic stones: maybe shards of obsidian, chunks of granite. [note in passing: why is he carrying around a worthless bag of rocks like this? An earnest question!]

When he arrives in the other country, the horseman empties his pouch and finds sand, which he discards, and a collection of well-polished and useful stones: the obsidian can be fashioned into arrowpoints, say, and the granite can be used for hand tools. They shine.

Group practice is like this bag of rocks, shaken around mercilessly, with no escape: each occasionally agitated participant wears off the others' rough edges, which fall away like sand from the eyes upon awakening, until everyone whispers and glides among each other harmoniously, smoothly, becoming useful in the process. The teacher, in the analogy a horseman, contrives all this for a reason. As I asked before, what is his reason?

My point is that sangha offers an opportunity to learn and grown that individual practice cannot provide. It draws out hidden sides of one's mindstream, one's karma, and makes them very visible and therefore workable. Positively, it generates deep karmic relationships over time that keep you involved at a helpful level in practice. It keeps you honest. It shakes you out of your habitual ways of doing things, facilitating your task of doing things another way. It helps you see how the aspects of Buddhist practice fit together in a coherent methodology (as distinct from the jumble of lists that some beginners perceive Buddhism to be). There are other reasons, too.