What are four valves under storage and water tanks for?

[Pedecanada]

My guess is for draining but on black and grey tanks which are up front why not just use big valve. Water tank which is farther back which is clean water tank also has two and a plastic drain on side also. This is on my 2009 panamerica also funny that two of the valves have the handles broke would they be hard to change or needed. I do live in Canada and will need trailer drained properly. Thanks for help.

[BigAl]

You better drain them right now. If they are frozen you are probably too late.
large valves are I assume the drains for the black and grey. The others are probably the low point drains for the plumbing lines.
Al