They're Fun
Kids love to play. If something is fun and exciting, they do it without complaining. In fact, if they really like a game, they'll want to play it outside of the "class" setting.
In the early grades, math lessons largely revolve around learning the basics of arithmetic: addition, subtraction, multiplication, and division. Even with creative teaching, there is no way to get around memorization, which usually involves lots and lots of repetition. This can make math seem extremely dull!
However, by incorporating games into math lessons, you can draw on your child's natural curiosity and playfulness to get them really interested in math. My son doesn't really realize that he's reviewing his math facts ... after all, we're playing a game!
They're Educational
Kids learn by playing, and board games of even the simplest variety teach important concepts. Beyond the core facts that usually form the basis of any math game, these games also teach strategy, cooperation, turn-taking, and being a gracious winner (and loser!).
Most of the games I've designed also have secondary themes. These can be educational in their own right. For example, "King Richard's Ransom" has a medieval theme. It was easy to incorporate some basic concepts from our unit on the Middle Ages (e.g., about knights, castles, tournaments, and various medieval professions).
They Break the Routine
During our first year of home schooling, when our son was six, he loved doing the same thing over and over. It was best if every lesson had the same structure: we began by reviewing basic concepts, moved on to flashcards, and then did a worksheet.
The second year was completely different. Anything that smacked of too much "routine" became very tedious. At this point, I developed a variety of games and other fun activities to make the class less monotonous. The games were a big hit, and I've designed anywhere between one and three new games per year. Everytime I make a new game, my son can't wait to play!
They're a Nice Reward
I've now instituted a "Friday Game Day" in math. This is a reward for the hard work my son (generally) puts in during the week. If the effort isn't quite there during the other days, I sometimes casually mention that we might not have enough time to have Friday as a game day.
That's usually all the motivation my son needs to put the petal to the metal!