The most common reason is wear and tear. Over time your tread depths will decrease to a level that is no longer safe. The legal minimum depth across most of Europe is 1.6mm.
There are other reasons why you may need to replace your tyres. For example, tyres can age and, if rarely used, begin to crack. This can also happen if they’ve been incorrectly stored.
You may, of course, get an irreparable puncture or damage from ‘kerbing’ your vehicle. Also, if you’ve been involved in any sort of collison, it is important to check that your tyres have not been compromised.