What is the use of creating full text catalog in full text indexing
From BOL:

After columns have been added to a full-text index, users and applications can run full-text queries on the text in the columns. These queries can search for any of the following:


One or more specific words or phrases (simple term)
A word or a phrase where the words begin with specified text (prefix
term)
Inflectional forms of a specific word (generation term)
A word or phrase close to another word or phrase (proximity term)
Synonymous forms of a specific word (thesaurus)
Words or phrases using weighted values (weighted term)


A full-text catalog is a logical concept that refers to a group of full-text indexes. Catalogs makes it easier for maintaining full text indexes.