I know this may soung silly, but can someone tell me if so, how does it affect traffic ? (positively or negatively)
Won't affect your traffic.. what it will do is affect your google account. A sitemap should only be updated as much as the site is updated. If you resubmit it, it means there is something updated. However, even if you have multiple changes in a day, I highly recommend you only do this ONCE per day.
In fact, google doesn't care if you submit your sitemap a lot or not. This won't change the rate wiith which they visit you either, and certainly won't prejudice you. In fact, you don't even have to submit it. Just place the new sitemap on your site root, configure your robot properly, and google will update it by itself. Take a look at: http://forums.digitalpoint.com/showthread.php?t=313270
Thank you for your reply, i have a guitar tabs site, and we are adding content (around a 100 tabs everyday) it is a very big site, and it will eventually get bigger. does it make a difference for my SEO if i update my sitemap once a week instead of daily ?
Not directly. What it will do is add more page to Google (SEs) and in turn they will be returned in the SERPs. If someone now clicks on it, you get the traffic.
if you submit site map it is helpful for the crawler . once your site is crawl and indexed then there is possibility to come on the search pages and it can indirectly increased your traffic .
Adding a sitemap to a low traffic site will greatly affect the traffic. However, if your site is high PR and any new post gets automatically indexed, then the sitemap addition will not have a big effect. In fact, a PR 4 or high blog gets crawled daily and any new link or content is indexed. So a sitemap submission will not have a big effect in traffic. On the other hand, a PR0 or PR1 site is not crawled daily, so some of hte pages may miss out of index, and any traffic that could have come through them will not come.