Question

Why is it important to remove all the contents of your tubes before attempting to load a gel?

Answer 1

It's not, and I would advise against doing it.

Explanation:

You need to load the same volume of each sample to the lanes, however, I would not load all the contents of the tube. Two reasons for this:

1. Extracting all the material from the tube means you run the risk of introducing air bubbles and these will float out of the well and take sample with them.
2. Hopefully, you have centrifuged the sample to throw down any insoluble material before loading onto the gel (insoluble material can 'breakdown' in the gel and cause streaking of proteins). Centrifugation also returns the sample to the bottom of the tube after heating. If you then try and load all the material in the tube you risk loading the insoluble material.