Question
why might it be a good idea to make a flower garden with a mix of annuals and perinnuals?

Answer
Annuals bloom much longer, usually from spring to early autumn.  Also, annuals grow much more quickly to make a big, beautiful, colorful garden in a very short time.  Downside to these is you will have to replant every year.

Perennials bloom for a shorter time, a month or two.  Downside to these is much of the time, the plants will simply be foliage.  Alone, perennials can make for a dull garden once the blooming season has passed.

Most people have separate annual gardens and perennial gardens because annuals grow so quickly, they can choke slower growing perennials out.  One good way to get around having no blooming plants in a perennial garden is to buy plants with different flowering seasons.  Some perennials bloom in the spring, some in the summer, and some in the fall.  

If you want to mix annuals with perennials, you should buy larger perennials that stand tall so the annuals won't be a big threat to their growth.