Websites, specially those which are running on popular CMS's like Wordpress, Joomla etc., are fulfilled with additional plugins and other custom code which is not smoothly written. As a result server needs more time to read it and compile to show to end user. If fix it, site performance may be increased in times.
Less load time
Average site has about 60-80 files to be transferred to client to show page fully. This means to show page clients PC or phone need to create about 60-80 requests. Page load finishes when all files from server are received and executed. If less files will be sent to client then faster website will be loaded into clients device. Also Google is fighting with bad websites which requires a lot of network traffic, so if you will have well optimised site google rank will be better for it.