Good Backlinks serve multiple purpose :
Everybody on internet is publishing content on internet. Search engine wants to know who has authority on content. Backlink tell search engine that source of content is article where link points to.
Even if new article is written such as "How to design a website" and author originally read other articles and took ideas, author will describe and give credit to original source by giving backlinks. More backlink help search engine determining authorative sites/authors.
Backlink also act as advertisement as persoh reading it get to know about where backlink points to.
Backlink transfer traffic to other website if user clicks.
Things not to have in Backlink
Backlinks were so important that comapnies spent millions to have references from other site. Google did number of improvement to determine good and bad backlink. Now it not only ignore bad backlink but also penalize it.
If you have a article about "design web portals" and it give backlink to "gold and diamond shopping store" , Search engine determine that there is no relation between source and destination. There is no genuine reason to give refernce. It treat this as artificial link and ignore, penalize it.
If a page has too many backlink, it is sign of bad backlink.
If there are many backlink on sidebar, header,footer of page. It means same backlink is appearing on many pages of sites. It is not referring to any page, it is simply giving irrelevant backlink.
A backlink that is natural. That is places in middle of an article. Article giving backlink and articlegetting backlink should be on similar topic and have overlapping words.
We provide good backlinks that are natural. These are built by person. Link is given from page that is on same topic where backlink point to.