Some people end up buying and peeling an onion and a head of garlic, when they only need a portion of that for their recipe, and end up tossing the rest, only to need it again a few days later. Freezing a puree of garlic, onion and olive oil means that not only do you avoid this wasteful routine, but you can prepare several months' worth when the ingredients are on sale.
Culinary weblog Vegetarian Perspective came up with the idea originally to placate children who were picky eaters. The adults wanted their food to be flavored with onions and garlic and pureeing them gave the flavour without the bits of onion and garlic for the kids to pick out.
The other thing I like about this method is the time savings. Instead of spending five minutes peeling and chopping, I just remove an ice cube or two of puree and let it thaw in the pan. Sure you may only get 75 per cent of the flavour, but for busy weeknight meals it gets the job done. The author of the post at Vegetarian Perspective didn't have any ice cube trays handy, so they poured the puree into a cake pan and scored the frozen mass into sections as soon as the puree begins to freeze.
True gourmands will probably scoff at this method, but I think it's worth having in your culinary arsenal.
Frozen Garlic and Onion Puree [Vegetarian Perspective]