What are they? 

BTECs are a more vocational qualification, which at Level 3, are considered equivalent to studying for A Levels. BTECs are more likely to be coursework based, rather than exam based like A Levels. BTECs typically take two years. You are graded Distinction*-Pass. The expectation is that BTECs will eventually be replaced by T Levels, but that does not mean they are a lesser qualification, and this change is expected to take a few years still. 

Why? BTECs are less focused on the academic side of a subject, and instead are hands-on, practical experiences to prepare you for the world of work. This can be a massive selling point for students who want more practical learning. You can combine some BTECs with A Levels, and BTECs are widely accepted by universities (all entry requirements for courses can be found on the UCAS website).