Will it help me reach more people? If yes, how?
Does that help in SEO too?
If you use social media buttons on your site, it gives your users, flexibility to share their interest in your site with a click. Unless your site has content that can not be forgotten, no one will bother finding your page specially, on the social media sites they use. Therefore, it will also give you more interaction opportunities with your visitors. As they will likely to see latest happenings in your site without actually visiting it, and a good content will definitely compel them to pay a visit to their site. They might even turn out to be your representatives as well in their social group. Although it largely depends upon the quality of content you post.
Furthermore, these social media sites employ some sort of tracking mechanism to these buttons(like button, share button)(in context of facebook), so that when a user often visits your site, he/she will also see your page as recommended in their respective social media profile or timeline page, therefore much chances of involving with your page.
So, all in all, do employ these buttons, but with a little bit of caution. Too much use of them will destroy your design. Identify, what major social media platform your targeted visitors use, and then only add widgets from those portals only.
Social buttons allow and encourage users to share your content. Without it, it would be too much trouble to share your content and most people won't bother. The more your content is shared, the more users it reaches and the more opportunity you have to capture new users. Social buttons along with SEO are the single best things you can do fir traffic short of Tweeting, E-Mailing, and otherwise sharing/marketing your content directly. It allows others to market your site for you.
No, you don't need social sharing buttons as people are fed up with all that social websites anyway. Only a very little amount of people actually uses them and they sort of destroy your individul design. I would suggest not to use them, but offer a comment section instead. People will have much more courage to comment your post when they don't have to do it with their personal account.
Answer: Because everyone else does it and social networks are obviously awesome. :)
Perhaps Alt Answer will be true for your site and perhaps that supports whatever 'better' is for your site. So, put them on there and track their usage with Google Analytics. If they're working, great. If they're not, do something different; put them in a different place in your layout, try a different logo set or get rid of them. Specifically, track the overall goals of the site and then measure how the social sharing buttons impact those goals.
Personally, I think they're the scourge of the web and offer very little value to most smaller websites - especially when you see post after post with "Likes: 0, Tweets: 0". Plus, why do people want to mash up their brand with everyone else's logos? That said, if they're working for you and your business who cares what I think. :) Only way to know is to measure. And then measure again.
If you use actual like/tweet buttons, you can track with this: https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingSocial
If you use simple sharing links, you can use normal GA event tracking: https://developers.google.com/analytics/devguides/collection/gajs/eventTrackerGuide
Here's an example of normal event tracking for the newer analytics.js:
Note that you can replace any of those values with something more valuable to you. For instance, the title isn't really needed since GA will track that anyway.
re: Alt Alt Alt Answer: Consider the networks and how they relate to your audience and your content too. Science papers will likely get few if any Facebook shares but they could very well be active within certain communities on twitter or digg.