To normally access say two Raspberry Pi's, maybe a remote virtual private server, a media machine in the sitting room, etc you'd probably use SSH from a terminal, as well as say a VNC Viewer, and maybe more applications. And you'd have to do that from the admin machine you've set it all up on.
With Apache Guacamole gateway setup, you'd just use your browser to log in to Apache Guacamole, and from there click on which machine you'd like to access remotely, whether by SSH, VNC, or RDP. The effort is done once to set up the configurations, but after that you can access via the gateway from any location, or even grant others access. So you are not tied to one admin machine that has all your remote access applications configured (big bonus if you are remote or travelling, as you just need a browser to access Apache Guacamole).
See Apache Guacamole™
Apache Guacamole is a clientless remote desktop gateway. It supports standard protocols like VNC, RDP, and SSH. We call it clientless because no plugins or client software are required. Thanks to HTML5, once Guacamole is installed on a server, all you need to access your desktops is a web browser. Copyright © 2020 The Apache Software Foundation...