I use a VPN when I start looking at ticket fares to fly to Europe from Australia. Companies use and store your flying history and enquiry so they know I always fly in July, so the fare i will get will be higher because my internet footprint says so. However if I use another IP address even within Australia, they might want to attract me with a better fare and by the time I confirm and start filling my name and details, they can't change the fare.
If it ever happened to you, you start looking for booking flights or hotels and very quickly the price goes up, it has most likely to do with them knowing it is you than a response to other users booking.
Deleting cookies and clearing internet history doesn't clear all your data.
Using a VPN is absolutely legal and if you ever use public Wifi you should 100% use a VPN because it also encrypts your data, including banking or other sensitive details.
Other very legitimate use has to do with protecting your privacy, you want to go online and search for a divorce lawyer without having the risk to receive ads on that.
If you think in 2019 that what you do online doesn't matter because you are doing nothing wrong you are very naive as I was. And I have to thank my DD for opening my eyes.