If you plant basil right next to your tomato plant - you can help prevent the pests that plague your tomatoes.

Plant a row of sage along with your row of onions - enhances onion flavor and repels nematodes.

Yes, I plant my veggie garden (every year) with companion plantings.
There are other "companions" that keep the bugs-at-bay.... radishes, nasturtiums, tansy, asters,
marigolds, even petunias. Come on out to the farm, we'll chat.