Constants are helpful in a few ways.
Like a normal variable, you can use them to make your code more readable to others. You can also use them so that the value is defined once in the code and then used in multiple places, if you have to change the value later, then you only need to change one line of code.
Previously constants were used for speed as they did not require extra memory lookups at runtime to retrieve the value. It seems after a quick google search, this no longer seems to hold true for most languages. Not sure if they are faster or not in GDScript.