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As Striker said, it's so you get down to a fainter magnitude.
I often take 10 minute exposures (15 x 10minutes is a common combination) where I get galaxies down to about mag 19 and 20, stars to about mag 22 or 23. If I am lazy I only do 3 - 6 minute exposures, but I end up with a much worse signal to noise ratio.
The problem is the tracking requirements. However, if you can do an exposure for 3 minutes then there's a good chance you can do a 6 and 10 minute exposure too. I found once I broke the 3 minute barrier the conditions didn't change, I just needed to keep going longer. By the time you're at 3 minutes you're already having to deal with PE and other issues, they just continue. DEC drift might not occur in 3 minutes and might in 10 so that could be an extra complexity requiring autoguiding in RA and DEC.
Oh, I should add: There's some assumptions here on what kind of stacking you use. If you average your exposures, then you need longer exposures.. so:
5 min x 10 or 10 min x 5 will give you a different result when averaged. The 10 minute composition will show fainter objects.
However if you do the same 5 min x 10 and 10 min x 5, and add the exposures insteads of average them, you will end up with much closer to the same magnitude level between them. However you will end up with much more noise.
Sometimes what I do is 20 x 10 minute. Then I add stack every second image, to end up with effectively 10 x 20 minute exposures. I then everage those 10 "exposures".
This combination of adding and averaging only works so far. Obviously if you don't have it on the chip, it won't magically come up in the add of the two 10 minute exposures. So if you actually did a 20 minute exposure you might end up with something more not in your exposure, but the objects that are there, will be much the same brightness in the single 20 min and the combined (via add) 20 min.
Hope that makes sense!
Roger.
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