Under normal use WordPress should not require FTP to install plugins or apply updates. If your WordPress site is asking for FTP login credentials, this indicates that advanced write permissions are not enabled for the site in Plesk.
To enable advanced write permissions, login to your Plesk panel, click on Website & Domains, locate the site and then click on the Hosting Settings link. You can enable advanced write/modify permissions from that menu and then click OK to apply.
