A stock is only compiled into your plugin (.amxx) if the function\variable is used. For example, suppose you have 3 stocks in your plugin\include file: get_name(), get_id(), get_health() and in your plugin you only call get_id() out of those 3 (get_name() and get_health() are never used). get_id() is the only function that will be compiled into your amxx file. This will, as said above, improve the memory footprint and reduce the file size of your compiled .amxx file.