It's mostly big bloated companies (no names mentioned, we know who they are) trying to capitalize on people's ignorance.
At the end of the day, there's no anti-virus protection against user incompetence. if you download, run and grant root access rights to a piece of software you don't trust, then you're setting yourself up for trouble.
I could go on and on in agreement with some existing threads about how Linux (what Android is founded on) is very secure, uses true multi-user access etc. in contrast to windows, Yes, this does protect you from alot, and yes, having read-only system folders also helps a great deal with this and to date, i am unaware of any real viruses out there targeted toward Linux that do anywhere near as much harm to said system compared to the mind blowing amount there are for windows. But
it is still down to the end user to execute common sense. Paid Antivurs for android is largely based on the nonsense theory that 'becoming more popular makes it more susceptible to threat' frankly a stupid claim made (mostly by jealous windows fans) for around 10 or more years now and has yet to become fact.
The fact that you need to purchase the full version further supports my theory, that they see a large market, and want to make a quick buck.
the most you need protecting from are fake apps put out there to steal your personal information, and 'lookout' seems to do this without the need of purchase. as for a virus, this is highly unlikely.
Tip: Read the reviews first