Learn CSS Grid by Building a Magazine - Step 23

No issues with code, but just another random code question from yours truly.
When we’re assigning values to attributes, why is that only some of the time we fill in spaces between words with dashes?
For example:
<img src="www.freeCodeCamp.org" alt="first-source">
<img src="www.freeCodeCamp.org" alt="image of a calculator project">

the alt attribute can take whatever text you like.
Dashes or not.
It is only meant to be a description of the image so that people who cannot see it, can hear or read about it.