Your iPhone screen is cracked. Should you still use it?
Yes and no. You can still use it as long as the surface is smooth. What does that mean? If the cracks are more internal where it doesn’t affect your fingers and won’t cut into your skin, then it is useable. However, once it starts to break your skin, you should get the iPhone screen replaced as it will only cause skin damage and possible irritation.
In regard to functionality, a screen break due to a drop may affect other parts such as touch screen functionality, home button stiffness, or other iPhone issues. If that is the case, you should fix your iPhone and make sure everything else is working properly. The longer you wait, the more time parts may shift around and cause worse problems.
As per your vision, this is up to you. Some folks don’t mind looking at a spiderweb of cracks on their iPhone screen. This drives me crazy personally. Your vision will not get worse, but over time, could cause headaches and irritable moods by looking at such an ugly and jagged screen.