A dedicated IP address is an IP address that is solely used for one sender. The main reason to use a dedicated IP address is deliverability.
If you share an IP address with other email senders, your deliverability score is influenced by everyone sending emails from this IP.
One black sheep ruins the deliverability score for everyone else using this same IP.
Another reason is performance. Sharing an IP address normally means also sharing the same email server. Depending on the sending activity of other users, your sending performance may suffer severely.