I tried to find a good link to explain receiver desensitization (desense) but I couldn't find one. They just talk about it as though everyone knows what it is. So I'll take a stab at it. If I say something wrong I'm open to correction.
An amplifier has a maximum output. Increasing the the input signal above a certain level will not make the output go higher. An amplifier with such an input signal is operating in saturation. If a very strong signal is present in the passband of an amplifier and there are also weaker signals in the passband, the strong signal essentially steals power from the weaker signals thus reducing the gain of the amplifier for the weaker signals. The weaker signals are said to be suffering from desense.
This is an insidious problem because it's not obvious that weaker signals are suffering from desense. Strong signals can be received but weak signals have a reduced SNR or cannot be received at all. It's a very difficult problem to diagnose without being able to measure the signals on a spectrum analyzer and knowing the preamp maximum output. Preamp manufacturers do provide output numbers but they are not standardized in any way. Tinlee gives the saturated output for 8 input signals. Winegard gives the maximum input signal in uV which is useless until you convert it into dBm and add the preamp gain.
When one reads comments about preamps being overloaded, this mostly means desense. It could mean 3rd order intermodulation problems but that's another topic.
Desense is the main reason that preamps don't work well in strong signal environments. One signal can drive the amplifier into saturation. Even if one signal is not enough to do it, multiple strong signals can drive the amplifier into saturation because the summation of the individual signals can exceed the maximum amplifier output. San Francisco would be a prime example of this. All the Sutro and San Bruno stations might come in fine but few if any other stations can be received because the amplifier is in saturation. Even the TV tuner itself can suffer from desense if the signals are strong enough. This is why preamps are not recommended in strong signal environments.
When I first put up my antennas I had a very high gain preamp and I had all kinds of problems which I traced to desense from KUVS on channel 18. Now that I know more I've been able to select a preamp that KUVS doesn't drive into saturation and I'm using a notch filter just before the TV to reduce that huge signal from reaching the TV tuner. All the other signals in the UHF band don't sum to be as strong as KUVS by itself!
The ideal solution would be to use a preamp with a tuned input, one that passed only the channel you were tuned to, but that would be prohibitively expensive for a consumer TV installation.
I believe the assumption is made that if you have very strong signals that you'll be watching just those channels and not trying to watch weak channels. If you have only weak channels (fringe area) then you can take full advantage of the added sensitivity of a preamp.