How good the seed is that you buy is very important. Seeds have limits too. Some seeds will stay good for many years, but others will only grow if they are planted in the first year. Some plants even make seeds that need to be planted within a couple of weeks or they won't grow. There are always budget concerns, and if you still have seeds from last year, you might want to use them if you can. There is a simple test you can do to make sure that old seeds will grow well.