We buy your house for cash - whats the angle ?

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Just got this in the mail, an advertisement from MontrealHouseBuyer.ca

They're offering cash to buy your house. I'm trying to understand what's the angle here. 

Are they basically buying properties off of desperate people for -10, 20% under market value ? It's seems hard to believe that anyone will want to sell their home under market value, especially in times like this where it's a seller's market, and people are paying above the market value in most cases (At least here in my city: Montreal, Canada). 

What am I missing here, what's the angle, what's the catch ?