One of the reasons why I started a tomato growing web site was to answer the "Why" questions.
When I first started growing tomatoes, every book I could find on the subject would say do this or do that without explaining the reasons why. After some years of experience and making a lot of mistakes, I can now answer some of the why questions which gives me more freedom to experiment.
After the rain we had last night, it got me thinking about the difficulties of growing tomato plants in compost that is too wet.
The first thing is that soaking wet compost contains very little air, and air is essential for good root growth. Secondly, plants use the moisture in their compost to absorb nutrients ... the more wet the compost is, the more diluted the nutrients are. That means the plants may not be getting enough food and also explains why if you over-water tomato plants, the tomatoes have less taste.
Another of those "why" questions is: why should I not feed plants until their first pea-like fruit appear. The reason is by feeding before plants begin to fruit, you extend the flowering period which delays fruiting.