Research – What’s a good way to uncover why users ‘need’ your product?

What would be a good way to uncover why users need your product? On a different context from a user or a persona's goals and needs. I am working on a product already in the market with many users. I want to research what actually 'triggers' them to look for our product in the first place.

Currently I am considering interviews with current users and quick in-app surveys to ask for their true intention for downloading. I am also considering a proper survey emailed to users who just signed up, but I'm not sure how to phrase the questions (I'm a newbie). Can anyone advise on some strategies or point me to the right direction on this? Thanks.