Basically, to make sure that everything is square and as parallel as possible. For example the x-axis and build plate you can align this way: Milling the cnc wasteboard flat (or so I thought): mis-aligned z-axis modules - #16 by brent113
If you read through that topic, you’ll also see I made the mistake of not doing it and due to making the mistake of turning of the machine and restarting, the x-axis and the carriage got aligned differently. The most important thing is that you have a reliable/reproducable way to align them.
If you ever run your tool head in the bed hard, break a bit, change tool heads, there’s a reasonable chance that both z-axis might might shift slightly compared to each other, and then it’s not aligned anymore. The same applies in the Y-direction but that’s not that easy to do without additional rails or something.
But in essence, if for example you found that the left side of your bed was consistently 1mm higher than the right side, then that might be an indication that both z-axis weren’t aligned perfectly.
Finally, there’s the chance there is some play on the x-axis (if you wiggle your toolhead it might move a bit forwards/backwards or left to right). If you wiggle hard enough it will anyway, the machine isn’t that stiff 
So whenever you make a cut, due to the resistance it might be pushed crooked just a tiny little bit. As your bit is 1" wide, a very small deflection like that can cause a little ridge already.
So how big are those ridges? Based on the picture it doesn’t look that big.
As already mentioned by @sdj544 running a second perpendicular pass on it, or do a spiral from the center outward might clean that up already.
Finally, you will almost always see a toolpath if you look closely enough especially just after you ran your job.
And then, for your wasteboard, if there is a 0.05mm ridge, that won’t really matter and for all practical purposes your wasteboard will be flat. The most relevant thing to look out for is that you can reproduce the parallellism between your x-axis and the carriage in a reproducible way so that when you’re changing toolheads etc you don’t come to the conclusion you should mill it flat again.
Obviously, since you already milled it now; the easies way is to get them aligned again is to just push the x-axis down onto the board (possibly with a 123-block or spirit level or something else that you know is flat in between as the toolhead won’t allow you to push it down completely).