a website that has a service or a some kind of tool so no content , I wonder is linkbuilding [and SEO in general ] usefull to promte it or its just a waste of time
Yes, It will help you but you need to add relevant content on your website first and decide about your targeted keywords and write you title, meta and content accordingly then start link building to rank for those keywords.
so you think even if the website is not 'searchable' (i.e. the potential users would not use search engine to find a service like what I'm providing) I could make it searchable and get traffic from SEs? will this work and give decent visitors more than other marketting ways which I think are better than SEO for web apps? I mean will SEO be better than other marketting way even with web applications ?
If potential users/interesters will not use search engines to find your services then don't think about seo go for social media (Facebook,twitter, Google +) and PPC marketing. Search for relevant communities (forums,discussion groups) online and get involved to market your services/products.
m_wc seems to have it right. However, link building and SEO in general are going to help. And maybe more than you think. You should keep an open mind and pursue any avenue that will bring in potential targeted traffic, no matter whether you're selling a service or a product of some kind. And yes, you do need some content.
Just my two c but i agree with thom02 link building will help you get traffic no matter what. Though i do somewhat disagree on adding content.... it will help you rank faster but it can rank without content, its just harder.