Here's an interesting blog post by Registered Dietician Lily Nichols.
Quoting the post titled 5 Reasons to Ditch Low fat Salad Dressing:
"You need fat to absorb the nutrients from your vegetables – That beta-carotene found in carrots is fat-soluble and so is the lycopene in your tomatoes and so is the zeaxanthin in your kale… (I could keep going). This means you must consume it with some fat to get the benefits of these powerful antioxidants. Also, the fat soluble vitamins – A, D, E and K – require fat for absorption."
It turns out this has been studied. This study found "a substantially greater absorption of carotenoids was observed when salads were consumed with full-fat than with reduced-fat salad dressing."
This one found that "adding adding avocado fruit can significantly enhance carotenoid absorption from salad and salsa, which is attributed primarily to the lipids present in avocado"
Plus that low fat salad dressing taste's terrible and we all could use another excuse to eat avocados:)