Its all about the Benjamins:
Here is a really simplified example. Suppose you are a large company generating $1B in revenue, and you have a market cap of $5B. You want to build an important new product that your CTO estimates will increase your revenue 10%. At a 5-1 price-to-revenue ratio, a 10% boost in revenue means a $500M boost in market cap. So you are willing to spend something less than $500M to have that product.