Division with mix off/def weapons

The default division models all come with a mix of attack and defence weapons, whats the point ?

if a land formation attacks a country, it enters the teritory, and can be attacked by enemy formations ? in this case only defensive weapons fight ?

That is correct. Offensive weapons can only attack and defensive weapons can only respond to attacks. Offensive weapons do not defend themselves in attacks that they do not initiate, which is why you would put some defensive weapons in an offensive unit.

Example:
If you have a unit with 1510 Heavy Tanks and no defensive weapons, and I attack it with 1510 Heavy Tanks. Your tanks will not fire back. It's stupid but that's just how it works. You would need to have some defensive weapons mixed in with your Heavy Tanks in order to not get massacred by my attack.

Hope that is helpful.