But have different roles in the strategic calculus.
Land-based missiles are a first strike weapon. The enemy knows exactly where the silos are, so if you don't launch before enemy missiles impact you will lose your silo ICBMs. The most extreme version of this is "Launch on Warning," where the silo missiles are fired more or less as soon as your IR satellites watching for launch plumes start screaming. A more relaxed version can wait until the long range radars get a track on missiles. About the latest you can wait is ~1min from impact.
Submarine-based missiles are second strike weapons, guarantors of MAD. Because even if the enemy launches and takes out all your bomber bases and silos, there are still missiles with enough firepower to end whichever country launched first. Submarines are very hard to detect, but advancements in underwater detection capabilities makes the guaranteed MAD a little less likely.
Bombers are a very effective external signal of alert levels to your neighbors. People see and hear them, they're tracked on radars from well beyond the horizon, etc. Especially when you have dedicated bombers. It's easier to miss the signals if you only use fighter-bombers.
