Companies which opt for free trials do so because they are confident that their product is going to make a difference to their customers, who eventually, after the expiration of free trial phase are likely to buy the product because of the promise and value they see in it, after having tried it.
Free trials help a prospect gain familiarity with the product, its features; not just in a sandbox environment, but right in the production environment. If the prospect does not like something in the promo run, prospect can voice his opinion and feedback, which can find its way back to the vendor company, who may just be able to accommodate the suggestions in the paid version.
