IN WITNESS WHEREOF....

I'm reading some document in English
Teachers!
I have two questions as below.

1) IN WITNESS WHEREOF, the parties hereto have executed the agreement.
---I'm not quite clear about the meaning and purpose of "in witness whereof".

2) as set forth in the above-described lease.
--- In the phrases above, is the usage of "as" correct and normal?
I don't remember that I have seen that usage.

[Tdol]

It means that the agreement recognises something- but without knowing what has come before, I cannot say what.

In 2, the use of 'as' seems normal to me.