Not every business is alike, so when you go to a conference you will generally find that people are approaching every problem from a different perspective. This can be great for varied and interesting discussion, but every so often you will find that you need help thinking about how a certain session will benefit your company.
Taking someone along means you will be able to discuss things with a focus on your firm, or at least the sector your work in. This way, your conference experience will be tailored more around your company and will therefore be more relevant to you.
You can be in two places at once
If there are two sessions you wish to attend at the same time, you can avoid having to miss one by bringing someone else. You can each go to a different session, then meet up afterwards and compare notes. This means you will be able to get more out of any conference.
You can get double the networking done
Networking is an important part of any conference, but your ability to meet new people is hampered by the fact that you simply won't have time. With someone else from your company at the same conference, you can meet twice as many people and share contacts afterwards.
Bringing a conference buddy along can be incredibly beneficial, and is well worth the extra cost. Just make sure to have a thorough debrief with them so you are sure not to miss anything that the other has experienced.