People generally don't know the $/share. Let's take the case where you only know the company valuation and the number of shares you have.
Eg: You know a company sold for $30MM and that you have 1MM shares. How much are your shares worth ?
Unless you know the outstanding shares you don't know if the price/share is $15/share (2MM Outstanding) or $3/share (10MM Outstanding) or some other number. In this case your portion could be worth 15MM or 3MM depending on the number of shares outstanding.
The percentage ownership lets you calculate the value of your shares directly through simple multiplication ie. your %ownership * Sales Price = Your Portion's Value
It also tells you number of shares outstanding. You can divide the number of your shares by your percentage to get shares outstanding.