before a sample is loaded on the gel,why it has to be prepared in 20% sucrose solution or glycerol?

Glycerol and sucrose make the sample more dense than the sample buffer, so the sample will remain in the bottom of a well rather than floating out. The difference between glycerol and sucrose is based upon their viscosity. Glycerol is superior to sucrose as a viscosity agent but Sucrose is better than glycerol for increasing density, because sucrose is less sticky

We used Glycerol in our lab and I found it much easier to work with than sucrose.

TO keep the sample heavy so that it can be at the bottom of the well and not floating