Was told I need a water pressure valve on the outside of my house. The house
Was built in the mid 80's and am wondering why I need one of these now?

Are you sure it is a pressure valve? I have never seen an external residential one in any place I have lived. How is water pressure regulated at your house now? I will assume city water. Do you see one of these in your basement or garage?
https://tse1.mm.bing.net/th?id=OIP.M152e29dd68288380fbd127b5c25dacffo0&w=85&h=105&c=8&rs=1&qlt=90&pid=3.1&rm=2 (https://www.bing.com/images/search?q=exterior+water+pressure+reducing+valve&id=F234E3FEBF66667E072E3BB30566180EF1A0F635&FORM=IQFRBA)
Who said you need such valve? I could see a check valve, these are typically at the meter.

In my area, every house has to have Water Pressure Regulator installed - if incoming water pressure exceeds 75psi. Even, if your incoming pressure is 75psi and less, you should still have one installed as all plumbing fixture in your house are designed to work at around 55psi. Water Pressure Regulator will allow you to reduce pressure to 55 psi and maintain it at that setting

Hope that helps

Milo