The first advantage is that liquibase keeps track of which SQL scripts (which in liquibase parlance are known as changesets) have been deployed to any given database instance, which if you are doing things manually you will need to do yourself. Liquibase also keeps track of whether or not a changeset has been changed since it was deployed, preventing other errors.
Liquibase does not require you to use XML to define the changes - but if you do, you also get another advantage in that your changes can be slightly less dependent on any particular database engine's dialect of SQL - it is possible to write a single XML changelog that will work the same on PostgreSQL, Oracle, Microsoft SQL server, and more. Some teams use this to give them the ability to do local tests that require a database using something like h2 as the developer database, and then a "real" relational database in production.