When you get something for free, then remember you are the product.There is nothing called free lunch and if you are getting something for free then remember that you are the product if, you are getting a free product or a service.
When you get a free product while you purchase something then, mostly it happens that you are purchasing more than required. For example, you went to buy a pack of bisquit but on the counter you saw an offer, buy 3 get 1 free. Now instead of buying 1 product you took 3 in order to get 1 free pack. Here you paid and will now consume more than needed.
Similarly in case of services you get similar offers. For example, get you car serviced at this workshop and get car wash for free.
Again, in case of apps you get most of the apps for free but in turn you have to give the app all the permission to collect your data and location, which can later be used by the company to target you to sell something or sell the data collected from you and others to another company for selling their products or services to you.
It simply seems as business promotion but in reality a person becomes the product for the companies selling their products and services to you.