Git isn't my primary source control solution -- edit: corrected in view of useful information below --
If you accept pull requests and pull data into your repo from other folks without a asking them to assign copyright or sign a very open CLA then that means the copyright is now joint.
The ramifications of this tend to be that you cannot re-license the code without their permission, unless the original license says you can.
Example - your project is GPL, you accept submissions from a bunch of other people. You decide you want to release a closed-source binary-only commercially-supported extra-special version with plugins and hotsauce. You can't unless you get permission from everyone that's contributed to the project, because the rights you have to their submissions are governed by the GPL. With copyright assignment, you as the now sole copyright holder could do this.
Ramifications will vary by license and by what you want to do.