There's nothing wrong with watching tutorials online. I mean come on, its free for crying out loud! There's a lot of professional musicians who also like to learn from our fellow musicians on YouTube. When it comes to music, there's always something to learn no matter what level you're at. When you already understand how music works and are proficient at your instrument, learning from someone online is probably a bit easier for you.
But, there's an advantage to having a one-on-one tutor or teacher.
Getting started. If you don't know where to start, private or group lessons would be ideal for you. Your teacher can help you get started. One of the worst things you could do when learning anything is learn it the wrong way. Its typically hard to correct it later.
Critique. Your teacher is there to help guide you from beginner to expert levels. There are somethings that a video can't give you which is critique of your own playing. I've found that for instruments or audio production tools that I taught myself how to use, I have to find a professional (friend) to listen to me or watch me record or mix to get some feedback from them. Also, when mixing a song, there are 10 different ways to do the same thing. Why not have someone show you their technique?
Ask Questions. You can ask specific questions when there is confusion while you're standing next to your instructor. Of course you can email someone online and wait for an answer, but you'd be waiting.
Referrals. In a lot of situations, your teacher is probably a working musician. Sometimes, they won't be able to do a gig based on their schedule or (if we're being real here) maybe they require more money. If you have a good rapport with your teacher, they will think of you and pass your name along.
I'm sure there are more reasons to choosing private lessons over taking lessons online. The thing to remember is that you aren't stuck with either one. You can take lessons for a while and then you can study online. You can also just throw yourself in a jam session and learn by making mistakes.... many downsides to that by the way.
But whatever you choose, PLEASE remember that neither will come over night. Where online tutorials may get straight to your answer, learning overnight won't happen. Put the time in and you'll be a thoroughly skilled musician later!