No, a drawing tablet isn’t strictly necessary. I’ve done things with the pen tool over pencil drawings scanned in, but it is a heck of a lot more work than using a tablet. The tablet is a tool that provides you better control over your strokes and curves; you’re just leveling up your tools to have better control and output. If you’re feeling limited by the tools you’re using and think getting a tablet will allow you to do a lot more, then get a tablet.
I don’t know if you play a musical instrument or not, but when I was looking for a new instrument myself, it was because I felt I had reached the limitations of my student model and I needed something to keep up with me. I tried out some intermediate models and it was a whole new world. The intermediate instrument allowed me to play quicker with better response than my student model. It’s a similar idea with drawing tools. Again, if you’re feeling limited by your current setup and a tablet would offer way more options, flexibility, etc., then go for it. I’m very happy with my current setup and I know I’ll be using it for a very long time.
There are some great cheap tablets out there, so take a look and don’t feel like you have to buy a Wacom, either. Huion seems to be another competitor with very reasonable prices for the same specs. Keep a look out for pressure sensitivity, programmable keys (if you like having a quick shortcuts), lines per inch (1000-2000 LPI seems to be enough, here’s why), and get the biggest active area you can for the price and portability so you won’t have to make cramped movements.
I hope this helps and happy hunting!