zcat is a tool that creates a pipe from gz file. It makes command cleaner and faster (maybe). You don't have to decompress gz file before using next tool.
Examples :
Finding string in gzip text file
$ zcat secret.gz | grep '42'Importing SQL backup file
$ mysqldump -u root -p db_name1 | gzip > db_name.sql.gz
$ zcat db_name.sql.gz | mysql -u root -p db_name_2