End caps fill the ends of frames, and round over sharp edges.
Caps sometimes slip out from the ends of tubes in projects which experience a lot of forceful motion or vibration. Same cap should fit both tube and extrusion.
End caps add a small amount of length to each end of square tube frame. This can be useful when accounting for the kerf of a saw used to cut frame to size. Allowing capped frames to reach exact length despite the removal of some material with each cut.
End caps can contain useful information such as plastic type for recycling, color coding of frame length for easy identification, and branding or logos.