The more information third parties have about a consumer the more accurately they can profile what causes a consumer to make decisions. Simply knowing what app's are on a mobile device can reveal a lot of demographic information about the user (think dating apps, news apps, religious apps, type of games purchased, etc). Likewise the type of hardware you are using can also imply economic information (using a $1200.00 phone vs. the most popular free phone given away with two-year plans). This type of information is worth a lot of money to a variety of organizations as it can help them persuade the consumer to do things (buy things, vote on things, change the way they view a political topic, etc.) What you are asking about is a HUGE subject and the data is used in millions or ways but the following to URLs would be a good place to get some background information on it.
BTW: a popular book which covers this information and probably provides a much more in-depth answer to your question is
Data and Goliath: The Hidden Battles to Collect Your Data and Control Your World
by Bruce Schneier