Whatever stage you are at with the piano you can always benefit from piano lessons.
Piano lessons can teach you new skills, techniques and playing styles whether you are a beginner or just playing for fun. The piano is such a versatile instrument that it's virtually impossible to ever know everything about it and the styles of music you can create with it.
Piano lessons can introduce you to new styles, like the blues, jazz, punk, rock, classical, baroque etc etc. If you've just started learning to play the piano then piano lessons are a necessity if you intend to get the most out of it. It's easy to lose interest in playing the piano if you don't see progress and piano lessons are the way to make sure that happens.