When you put an antenna in the air, you have to think about how many transmitters are within range. In a big city it can be hundreds or thousands. Every one of these signals goes down the coax to the radio. You may be trying to listen to a weaker signal, and many of the unwanted signals can be very strong. Your radio is expected to select your wanted signal and reject all the others. If the signals are strong enough, or close enough in frequency, at some point the radio can no longer reject the strong signals and hear the weaker one.
All transistor amplifiers have an upper limit on how much power they can handle. If a signal or bunch of signals get to this limit the transistor starts to distort the output, and the amplification goes down. The distortion causes signals to mix and create new signals on new frequencies, and this is called Intermod. Intermod means intermodulation distortion. When these new signals are on your RX frequency you experience intermod or unwanted noise and signals. The amplification going down causes weak signals to also get weaker so they are harder to hear.
When engineers design a RF stage amplifier they have several things to consider. One is the amount of current the transistor draws, and another is the ability to amplify strong signals. On a handheld, battery life is quite important, so the design favors low current drain, and low current means the transistor can only handle lower powered signals. This works okay when the antenna is a rubber ducky and all signals are quite weak, but if you put a good mobile whip or base station antenna on a handheld you will probably get distortion and hear several unwanted signals.
Mobile radios are better than handhelds because current draw from the battery is not as important, and they are designed with better transistors, but seldom the best. They will still suffer from intermod, especially in large cities where there are more transmitters.
Commercial base stations and repeaters have the best immunity to intermod.
If you are experiencing intermod it can be very annoying. It can break your squelch when there are no hams transmitting on your monitored frequency, and you hear many different forms of interference, from voices, music and general noise. One way to reduce this noise is to reduce the strength of signals you don't want to hear. This is what a bandpass filter does. It passes the band of frequencies near your wanted frequency, and rejects the signals farther away from your frequency.
You simply put the bandpass filter between your radio and the antenna and your intermod noise goes away, most of the time anyway. If occasionally the interfering signals are in the passband, or very near the edge they can still get through at nearly full strength.
The OCI filter rejects signals below 142 MHz and above 150 MHz by over 10 dB. This is usually enough to kill intermod. If there are strong signals between 142 and 150 MHz they can still cause intermod, even with a bandpass filter.
In summary, if you have intermod, a bandpass filter from OCI will usually solve the problem, and you will hear weaker signals on simplex better. The simplest way to find out is to try one and see.
Filters are especially good for mobile operation as your RF environment can go from good to bad depending on location.