As part of optimizing your web pages, you might modify content/designs, add /update articles, get more inbound links and the like; all of these need to be brought to the attention of search engines for faster reflection on your ranking index. Though search engines are self capable of revisiting your website once in every few months, this might not be adequate to get you immediate results.

Resubmitting the website after making a bulk of modifications to the website would be appropriate. The frequency in which such resubmissions are made does matter with most search engines. There has been cases where websites have often been black listed for submitting to search engines very frequently. I would suggest submitting a site to search engines, just once in a month would do the job.

Too frequent submissions to search engines would be treated as equivalent to spamming them and mostly every search engine now follows a 'NO-SPAM' submission policy.