Most of us don't have the $$$ to buy all of our produce organic. Some conventionally grown fruits and vegetables can be saturated with pesticides that can't be easily washed off. Others are less vulnerable, and are okay to buy when conventionally grown. I've eaten some conventionally grown grapes that had a noticeable chemical taste even after thorough washing.
If you want to know which are more likely to be contaminated by pesticides (hence worth buying in organic form), check out this article.