great question, which is likely to get a lot of responses.
Of course the angle at which you approach a corner is important. You set your corner entry point based on where you want to exit the corner, and where in the corner you want to be the tightest inside the corner (called the Apex).
If you enter wide you can exit shallow, but if you turn in too early you will push wide on the exit.
On the race track, you have to find "Points Of Timing" - these are physical reference points that mark activities like braking, turn in, etc., so that you can ride every lap the same and use intelligent experiments to improve track times.
There are a few Youtube videos that review it. Keith Code's "Twist of the Wrist" is an excellent read.
My personal experience, which may be different than others; in club races I found that I could brake later in turn in from a wider position than I almost always anticipate. As a novice I would always turn in to early and get pushed wide on the exit. When you leave a lot of room on the inside you can get passed on the inside, but you can keep your corner speed and exit speed up this way. Trail braking enters the picture, and helps keep speed on corner entry, but it isn't for the faint of heart.