Some benefits...
1) It can earn you backlinks which will help your rankings. People tend to no longer give out backlinks unless there is a good reason like "go here to download this really useful resource".

2) It keeps people engaged on the site and strengthens brand awareness.

3) It can greatly boost CTR from the serp by saying "free ebook" in the title. I'm not saying CTR magically skyrockets your ranking but more traffic to a website is a good thing. Who cares if they rank #3 if they have a higher CTR than the #1 site.

Some pitfalls...
1) Make sure you provide it in the best file formats for your audience. If you are pushing out an ebook, don't just provide a pdf version take the extra step and convert it to other ebook file format so it works with more readers.

2) It can eat up bandwidth. Bandwidth is relatively cheap but a popular uncompressed 100mb file can quickly add up.

3) Locking up content in a download can minimize its benefit. You probably want to republish that content via html/php so search engines can more easily access it and send users to it. Yes, search engines can send users to a pdf file but that pdf wont have your site navigation or ads.

Thank you :-)

I had two pages on a site targeting a set of terms related to advice. The page that I wanted to rank (a detailed guide) was being out-performed by a short companion piece about what NOT to do.

I added a downloadable template to the 'guide' page and re-jigged (i) it and (ii) the other page to (i) target / (ii) support terms with 'example' or 'template' in (as well as the previously existing 'guide' and 'advice' terms I wanted).

The page I wanted to rank subsequently performed better for all terms. It happened gradually (over a few months), which would indicate people being happier with the page as a result of the download, and Google noticing this over time.