You can blindly go out and and use tools to try and penetrate systems, web servers etc. without having to know how to subnet and what subnets are and their relation to an overall network.
Will knowing help, very much so. Even just when you're doing recon, you're going to come up with all sorts of IP addressess from various sources. Understanding if a host is in on an extranet, intranet, DMZ etc.. and it's relation to other targets is very helpful if you have a point and purpose to the attack (I'm looking at this ethically of course).
If one has access to an internal network and a specific host, which has and IP of 172.16.25.100/16 only, yet is trying to get to a server with valuable information which is on a totally different subnetwork with IP 172.25.242.254, then you'd need to understand how to get to this server.
Security aside, the simple fact that you are on a differnt subnetwork, even though within the same building, you won't be able to do much if you don't understand subnetworking. You wouldn't be able to Arp Spoof, because out of the same subnet, or maybe even ping it if each is pointed to completely different default gateways. I just woke up, so I'm a bit foggy lol, but I hope I kind of make sense...
Before launching a tool or attack we need to be able to access or reach a machine through maybe a simple ping to know it's alive, then see what its vulnerabilities are to know what attack to launch, which may not be possible depending on something as simple as different subnetting schemes.
A+, Network+, Security+, CIW Associate, CCNA, C|EH