Carol,
The reason a lot of breeders use shoe boxes, are so they they use a fungicide (usually methylene blue) on the eggs...without treating the whole tank. Methylene blue will stain the silicone...and can be be ugly in a show tank. Another reason for the shoebox...is so they can float the box in a heated tank, to maintain temp. Some breeders with fish rooms, heat the entire room. Those that don't, find it less risky to float the hatching container...as opposed to using another heater. In a small hatching container, temps are harder to maintain with an additional heater. It's too easy to cook the eggs.
I like it for easy water changes too. I pour of the blue water into a small bucket, and simply dip a corner of the hatching container into the water they're floating in. This gives you a fresh supply of cycled water...at the exact same temp. The bucket is in case you pour off any fry. I retreive the fry in the bucket with a turkey baster. One more reason for floating, is that there is usually an air supply nearby (for an airstone). The tank used for floating, probably has an air driven filter in it...so it's easy to run another line for an airstone, to the eggs.
Hope this helps
![Smile :)](./images/smilies/icon/smile.gif)
Rich