What does 所 mean in 正如我们之前所说? In the following sentence:

正如我们之前所说，Alphabet在企业投资者中似乎有些另类。

I assumed the usage of 所 in this case is, according to Pleco:

[as a noun without 的, esp. when the verb is monosyllabic]
据你所说

But in this sentence there is no preposition such as 据. The whole sentence would work fine even without 所 I think.
So what does the 所 work for and what meaning does it have, compared to the case where 所 is not used?
正如我们之前(说)= Just as we (said) previously- 说 is a verb here

正如我们之前(所说)= Just as (the words) we (said ) previously

"我们所说" is short for "我们(所)说的話" = "the words (that) we said" 

therefore, "我们所说(的話)" is a noun

When you say "正如我们之前(说)" you have to follow it with a quote of what was said

When you say "正如我们之前(所说)" you can follow it with a quote of what was said or omit it and presume it is known