Many summer vegetables will grow quickly in the long days of the northern latitudes. Some crops such as green beans, basil, cilantro, carrots, beets, sweet corn and more will produce and then wane and finish their productive life cycle. This is where planning for successive plantings comes in!
Take green beans, for example, they tend to mature in about 60 days and then produce a sustained flush for about 2 weeks and then they stop flowering and stop making pods. So, the wise move is to plant successions about 2-3 weeks apart for a continual harvest. For instance if you planted green beans on May 15th, then you'd want to start another wave on June 5th and then another on June 20th and maybe even a 4th planting after that for a sustained harvest of fresh pods.
In some climates, cucumbers and zucchini may become besieged by powdery mildew or other diseases so that their productivity begins to wane. In that instance, you would be well served to plant a later wave that is coming into productivity as they first planting is tapering off.
Other crops like cilantro, arugula and basil struggle with providing a consistent supply of fresh green leaves and the best bet is to replant more and then compost the early plants that may be bolting (flowering).
Root crops definitely require successional planting because the part we harvest, the root, requires the removal of the plant. We plant a spring wave of carrots, beets, radishes and turnips in March and then another in May and then a final fall wave in late July (for fall harvest and winter storage).