The message
from Frogleg contains these words:

"Companion planting" usually touts the benefits of
attracting/discouraging insects, but IMHO has mostly to do with with
plants that enjoy similar conditions.

There's more to it than that, concerning the chemicals in plants. Onion
family benefit some plants (like roses) but discourage others. Mexican
marigolds supposedly repel couch grass.

Another poster mentioned
tomatoes and basil -- they both do best in full-sun, warm locations,
and are certainly companions at the dinner table, but AFAIK they don't
encourage each other in any significant way.

I was speaking of tomatoes and basil grown together in a greenhouse.
In the UK, basil won't thrive except under glass; and tomatoes are as
often as not grown the same way, especially in the cool north. In the
enclosed environment of a GH, basil deters whitefly.

Janet.