A good flash is an essential tool in any photographers' bag.
The dinky ones on the cameras are borderline useless.
Most photographers who aren't shooting for money go in cycles. They try to shoot everything in natural light. Then they dump into flashes.
If you are shooting for money, unless it is forbidden (think certain sports), flashes are close to cheating, especially back in the film days. Vivitar 285 + Kodak tri X pan 400 asa = win.
I like flash. I don't like it when I shoot candids. It alerts people, 'oh, no - he's taking pictures again'. In that case, I use a long lens from across the room and high ISO to stop motion.
Big thing on flash, is GET IT OFF OF THE CAMERA BODY.
That's a rookie mistake. Without contorting the flashhead and using modifiers like bounce cards or tents, shooting with the flash directly over the lens and straight on makes for lackluster pictures.
Putting your flash at an angle, and varying heights off of the lens increases contrast. It takes a pic from meh to wow.
Yongyuo flashes are pretty decent. The newer ones talk to the camera, and they work out what is exactly enough flash, which is nice when you don't have time to figure stuff out. They also have a lot of advanced features you might never use.
You don't have to buy new, find a decent used flash. It doesn't have to be a monster, either.
The big thing is get out and use it. Everywhere, like how some people use salt. Try a pic with and without it.