In a pool you get steady small payouts in proportion to your hashrate. In solo mining you look for a block yourself: the reward is large (a full block) but comes rarely and unpredictably.
Over a long period, average income for the same hashrate is the same in a pool and in solo. The difference is variance: in a pool income is smooth, in solo you can have long periods with no reward. Solo only makes sense with very large hashrate (a noticeable fraction of a percent of the network), otherwise the chance of finding a block in reasonable time is low.
For most miners a pool is the sensible choice: predictable payouts and less risk of “never finding a block”.