Why does Urbackup client block the database when using xtrabackup?

There are scripts included in URbackup for performing incremental Database backups using Percona Xtrabackup.
However, I’ve noticed the database still gets locked while the backup is taking place.
The server I’m backupping has multiple mariadb innodb databases.
If I configure urbackup to use the mariadbxtrabackup script, the database gets locked and no writes can occur on the database.

Why is this so? I thought the whole point of xtrabackup was to be able to still use the database while a backup is being performed. If it locks the database anyways, whats the point? (apart from incremental database backups).

This is more of an Xtrabackup question, I guess. Googling a bit shows me that it locks tables that are not innodb, and for schema info. UrBackup buffers a bit, so that could exaggerate the issue. You could modify the script and use a file or mbuffer to add extra buffering, I guess.