Newman, differents between postman link and api collection uid

New to APIs/Postman
, ,
1

Hi. I connected my git repo with Travis CI, and in .travis.yml file I used:
https://www.getpostman.com/collections/xxxxx
Everything works fine but i want to know what differents are between using postman link and link like:
https://api.getpostman.com/collections/xxxxx?apikey=xxxx
Results are the same but in what case i should use 1st method and when 2nd?

Thanks for help