Often the whole is greater than the sum of its parts when it comes to planting.
The considered combination of colour and texture when putting plants together - both in flower and foliage - is something which is hard to do well if you buy plants individually and on a whim. Plants change through the seasons too which makes the task of choosing what to go where even more complex!
In any project we also have to take into account the mood we are after and what can work in the environment that we have. A plant grouping only works well when the group appears to be in context with the surrounding space.