I learnt and build websites years before i discovered Bootstrap. So I have built without it.

I currently use Bootstrap as my framework and I enjoy it because it makes things easier , faster, saves time, and when you are working with other developers on a single project, it reducing the frustration of them reading your custom codes & making adjustments.

In summary, do not overlook learning Bootstrap, you might be working on a group project that requires it. After all, it is currently the most used framework.