Running a speed test primarily tells you three things: Download speed - A measure of how quickly data or content can be transferred to your device over the network.
This test is performed by measuring the time it takes for the server to reply to a request from the user's client. The client sends a message to the server, upon receiving that message, the server sends a reply back.
There are many reasons as to why you should be regularly testing your WiFi speed at home; and, not for just the obvious instances where you find your WiFi signal strength is weaker than normal. Testing speeds on a regular basis allows you to pinpoint performance trends, isolate which devices may be causing issues for your wireless network, and ensure no one is stealing your bandwidth.