[bullsbarry]

The fact that aircraft get lighter as they burn fuel increases their effective range since a lighter aircraft requires less fuel to fly.

[edrxty]

It's also important as most long range airliners take off with more fuel than they can safely land with. The air frames are very heavily optimized as every pound matters. If you always landed with the same weight you took off with, you'd end up needing to make the landing gear and surrounding structure significantly stronger to prevent excessive wear over time.

[nradov]

Also landing speeds would have to be higher because stall speed increases with weight. Faster landings are more dangerous.