Absolutely. Other than generate keyword ideas, online keyword tools also display information you can’t get anywhere else: the number of monthly searches (both worldwide and in specified locations), cost per click and bid competition (for PPC campaigns), keywords used on competitors' sites, etc. All this data is vital for maximizing the quality of your site’s optimization, and it’s also prone to frequent updates which may require you to change the keywords you use.
For the same reasons, you should always use a keyword research tool to analyze your own keyword ideas.