Security in Office Buildings

I have had occasion lately to enter a number of office buildings for various reasons and the security measures vary enormously. This led me to wonder both how effective and how costly they are.
On entry some corporates have a reception and some do not - depending on whether the general public is likely to appear. Most office buildings these days have some sort of card access in the lifts, commonly with an additional point (using the same card) on the floor as well. I am including the independent barriers before the lifts in this as well. One place even had reception on the first floor - which is only accessible via a secure elevator!
I can see the reason for these - barriers ensure that security officers (if they exist) can see people, unlike security in the lift itself. With multiple people in each elevator it makes sense to have separate access to the floor - it is a challenge point for humans if you do not recognise the person trailing you. Depending on the corporate culture, challenging does actually happen although it is rare if the person looks like they know where they are going (whether or not they actually do).
As a  side-note: there is an increasing trend for new building to have lift buttons outside the actual boxes. I have heard that this is to increase security but I really can't see any obvious way this would be true. More likely it arises from centralising the control software to more efficiently allocate the lifts. Nevertheless I think it is a major backward step in usability. The design really doesn't take human foibles into account and I can see multiple ways the theory could break in practice.
Then of course there are the security guards and pass-cards for temporary access - some of which require a driver's license before being provided. All to ensure that no authorised people are allowed access.
And what are they protecting? An office building - that is - levels and levels of desks with some computers on them. Valuable equipment yes, but rarely critical. Who are they protecting them from? I don't know. I am not sure what the expected attack is supposed to come from or what benefit the attacker is supposed to gain.
Simple thievery could be deterred by much simpler and cost effective means - how much does it cost to have 3 full time security guards + reception staff and how often are they actually required to do anything. The alternative is prevention of attacks on staff, either personal or corporate motivated. However, there are surely simpler and more unobtrusive mechanisms for detecting unauthorised entry. If the police force, surely one of the most targeted groups does not require the same level of control then I can't imagine that the department of Economic Development is in great danger.
All security is a cost-risk comparison and I really can't see the benefit in those terms. So perhaps the reasoning is more about presenting a sense of importance to any visitors.