"border: 0px solid" in Build a Survey Form

Tell us what’s happening:
I didn’t understand why do I need “border: 0px solid”.
It’s because it’s 0px.
I checked it with and without it.

Thank you so much.

Your code so far

#submit {
  background-color: #59ace0;
  border-radius: 4px;
  color: white;
  font-size: 1em;
  height: 40px;
  width: 96px;
  margin: 10px;
  border: 0px solid;/*What for?*/

Your browser information:

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.100 Safari/537.36.

Link to the challenge: