Link building for SEO is important because it is still part of Googles SEO ranking algorithm. Reasons to find websites to link to you can be to increase your keyword search ranking, your overall website rank, sending high value traffic to your site, growing your authority on your topic, bringing traffic that may then amplify your site like some social media sites could.
You need to be strategic in how to get links. The first step is to create content that is worth sharing. You can go to ahrefs.com and http://buzzsumo.com that allows you to search by topic to see other sites who are getting links so you can see what kind of content already has high demand. Once you have identified good content, you can redesign it, update it, or expand on it. Majestic.com is a good site to see how your content is doing.
AuthorCrawler or http://www.tomanthony.co.uk/tools/author-crawler/ and blogdash.com can help you put together a list of websites you can link with. Create an excel sheet with the subject, domain, the reach it has, its authority and recent posts. Then you can put columns with your outreach efforts with dates and how you contacted them and the response.
When you contact people be of value. See what they may have as broken links on their website with https://chrome.google.com/webstore/category/extensions?hl=en-GB
You can tell them how your site fills a gap they have. Have a good subject on the email, and add your contact information including your site. Then you upkeep your links by making sure they stay working. You can use google search console’s Crawl Error report to search out any of your pages that return a 404 error. And the make sure your links stay active.