One of the main reasons is that you want to host multiple services on the same IPv4 and port since you usually only get one IPv4 (works for IPv6 too but there getting more than one from your hoster is a lot easier). This is known as name-based virtual hosting.
Another thing that is often (but not always) handled by a reverse proxy is SSL/TLS termination. That way the actual application doesn’t have to worry about the certificates or crypto-related security updates. Sometimes TLS is used again on the bit between the reverse proxy and the backend server but if they are both on the same physical machine that bit is often skipped.
There are also other services such as rate limiting, caching or fully featured Web Application Firewalls (WAF) and of course CDNs that come in reverse proxy form but you shouldn’t need to worry about those too much for a small personal website that isn’t used by thousands of users.