I've heard some DX indicate "CQ up 5" for instance, indicating that they are listening on a different frequency than they are transmitting on.
What are the reasons one might want to do that?
This is called operating "split" and it takes a certain amount of skill to work a DX operator working split and even more to operate split.
The reason this is done is to help manage the pileup. If too many stations are calling and the pileup becomes unmanageable it takes longer and longer to complete an exchange. This is less fun for everyone.
By working split, the DX operator keeps his transmit frequency clear so that the callers will hear him well making the exchange go more smoothly. He also has the opportunity to spread out the calling stations on several frequencies near his transmit frequency.
For instance he starts out simplex, then gets spotted and the rush begins. So he calls "Up 5" meaning call him on this frequency + 5KHz. After a while he might announce "Up 10" and realistically at this point callers are spread from +3KHz to +12KHz or so in their eagerness to get through.
The DX station will go through the calling stations in some order (usually). Perhaps starting at the top of the window and calling a station every 1KHz or every other KHz, or some other system. The key to navigating these waters is to LISTEN for a while and figure out how it's being managed before jumping in. This will greatly improve you odds of getting this guy in your log.
The convention seems to be to work split "up" on the USB bands and "down" on the LSB bands.
Another advantage of the "split" comes from the US Extra test material.
The DX station can call CQ on a frequency that may be out-of-band for broadcast for a lower license class (say General/Technician) OR for other countries which may have a slightly different band limit (but who can still listen to the CQ call frequency). And the CQ station can listen on the lower or higher split frequency which the responding station is licensed to transmit on. This allows use of additional frequencies in a crowded DX environment.
For example, DX station calls CQ on 14.220 MHz and listens "up 10" which is at 14.230 MHz. In the US, the General Class license is authorized only above 14.225 MHz - this would allow the General Class (and higher) responders to transmit on a valid frequency while listening on what otherwise only a higher US class would be able to transmit on.