Age differences between siblings aren't really all that plannable of course; you could get pregnant unexpectedly because lactation ammenhorea didn't turn out to be all that reliable a method of birth control after all, or the second child might take a while longer to conceive than you'd hoped. If you are thinking about child spacing or already pregnancy with an "Irish twin", though, here are some good reasons kids close in age are wonderful.
Get the tough stuff out the way
When I had tiny tots (let's say, newborns to about age three), being a parent didn't seem hard. It just seemed to be reality, which is what it was. Now that my children are older, I realize that some of the aspects of taking care of small, completely dependent creatures are indeed quite hard. The lack of sleep, the intensity of the whole experience, the wondering if you're doing something really wrong (like the time my oldest fell off the bed as a baby), the temper tantrums, the doctor's appointments... I could go on, but the point is that parenting really small children is a lot of responsibility, at a time when we're least experienced with caring for kids. Some moms find parenting babies and being pregnant especially enjoyable, but many do not.
Being pregnant while caring for a toddler also has its issues. You may not feel comfortable lifting your toddler, and may not physically be able to do so. You may also be cranky, tired, and suffer from morning sickness, something the toddler won't appreciate. As time goes on and children get a little bigger, you might be amazed at how independent they have become. You might appreciate how well they can communicate with you, especially once they start sharing their views of the world and deeper personal thoughts. At this point, parenting becomes more about having a relationship with your child, and less about changing diapers and being tired. That's nice. And having that niceness interrupted may not be very appealing. So having two small kids at the same time, when you don't yet know how much nicer parenting bigger kids can be, could be a really good idea.
Having two close together saves money
Having two kids close in age can save a lot of money, unless they're so close together that you'll need to buy two of most things. In my case, my son was able to use the stroller, crib, infant car seat and most clothes that had belonged to my daughter. At that point, I was happy we went with gender-neutral stuff rather than the pink stroller my husband initially wanted. Of course, my two-year old still liked to ride in the stroller and couldn't walk for very, very long. My son spent most of his time in a mei tai baby carrier when he was very tiny, while the stroller was mostly for the two-year old except when I wanted someplace to lie my son down while I went to pee or something. The baby did inherit a lot of his sister's clothes and toys as well. Because they were so close in age, we had not thrown anything away. Now that they're both older, most baby stuff has been given away. That stuff takes up a lot of space you could use for other things, after all.
They might fight, but close-age siblings can be a huge gift to each other
My two quarrel quite a bit, but they love each other unconditionally and would both do anything to protect the other. They are each other's best friends, and I am so happy about that. This alone is the biggest reason in favor of having two close in age to each other, even though there is never a guarantee that two close-age siblings will be as thick as thieves. My husband has one brother, and he's eight years older. They never really related to each other, though they will help each other in case of real need and are on good terms. I don't have any siblings, and I think my husband experienced his childhood in much the same way I did. His brother moved out when he was small, after all. I'm hoping my kids will support each other for the rest of their lives, in one form or another even if they end up living on opposite sides of the world.