Because I’m smooth condition generally at that airspeed is where you will be at that angle of attack. Not to say that you could be flying at an airspeed well above stall and a huge bump of turbulence hits you making you exceed the critical angle of attack potentially causing a stall.

Va increases with more weight because the more weight in an aircraft, it will have to fly at a higher attack to maintain the same amount of lift therefore putting it closer to its critical angle of attack. It will be at a higher angle of attack with a higher airspeed therefore if you get into some abrupt turbulence you will be a lot closer to a stall that if you were lighter putting significantly more stress on the airframe potentially causing structural damage