Why would I want to a use baseline grid in Illustrator cc?

tl;dr: the baseline grid is useful to align other, non-text objects to your text baselines to give your layout a very tight vertical rhythm.

A baseline grid is not just a tool for setting text in such a way that line-spacings are proportional and match in a single document. It also is a powerful tool in placing and aligning non-text objects in the layout.

One of the much-overlooked aspects of layout is that it’s not just important to ask yourself where you place an object, but also why there. A very good ‘why’ is ‘it aligns with something that was already there’.

Any layout, even one without any text, even an empty one, already has places to align to that are ‘already there’. The paper edge, the margin, the mathematical centers; the golden ratio or the Rule of Thirds if you want to go fancy an draw some guides.

I always say that I’m looking for ‘excuses’ to put something where I put it. All those examples I just named are perfect excuses to align to. I am always looking for more lines, more excuses to which I can align objects. The baseline grid gives me tens of new horizontal excuses.

If you use the baseline grid to align non-text objects to your text baselines, you will reinforce the vertical rhythm that has been dictated by the grid. A lay-out just looks that much better when the edges of images align to text baselines. If you establish a vertical rhythm with a baseline grid in your text, and then break that by misaligning the edges of non-text objects, that’s bad design. You have a multitude of aligning excuses in your grid. Use them.