For the purposes of monitoring users on a corporate network, the difference between on-the-wire sniffing/interception and proxies is small. In-fact, an on-the-wire box is a superior choice in most respects if all traffic is forced over it, as it will see traffic across all ports.
However, proxies are not solely installed in corporate environments for spying and blocking on their users. The abstraction layer they provide of requiring HTTP/HTTPS traffic (of which is likely the majority) is most definitely good practice to increasing the security of your corporate network.
Whilst alone, a proxy is not the be-all-end-all of security on your network, it is certainly not a solution to be ignored because "it doesn't do much". Assuming a windows dominate network, setting up group policy on your network to require clients to connect out on the corporate proxy with Active Directory credentials is a fantasticly simple mitigation. This prevents Mr.BadGuy from rocking up to your network and (a) sniffing internet-bound creds which can be used for elevation and (b) exfiltrating corporate data out of your network without even as much as an account.
A proxy in a well-designed network can prove to be an elegant solution and really, in a corporate environment, not much (if any) more than HTTP/HTTPS should be allowed outbound. Whilst an on-the-wire solution gives you a lot of power, it also brings along a lot of risk. How much capital would an attacker gain from breaking into your on-the-wire box?