What are the Pros and Cons of assigning value for ids from an offset? (Ex: 100,200,1000)
There are a variety of reasons why you may want an offset. Please be aware, some of these reasons are not necessarily good DBA practice.


As dnoeth mentioned, your primary key may be used in some other business process such as an order number. In that case, you don't want your customers thinking they are only the 11th order!  Being order number 1100011 is much better.
You may have a old system that is in read only mode.  It has all your old customers in it with their primary keys, and you want the new system to pick up where it left off.
Same idea, you know you will be importing PKS of a certain value at some point in the future, so you start with ones you know won't cause a collision.


Personally, the only time I offset is to avoid PK collisions. Sure, there was that one time when I was younger that I just though starting at PK ID 10000 was cool, but that was a long time ago.

Really, there are no Cons of doing so.  Except for some nominal space issues between data types and potentially losing the ability to assign PKs up to the data type limit, the database doesn't care if it starts at 1 or 1,000,000. It is going to index it all the same.