Can they just have 1 network and 254 host per net? why would a company use different networks?

There could be a ton of reasons, I'll just add one more. The more hosts on a network, the more work has to be done for each broadcast packet sent and the more broadcast packets on the network.

Say the average host generates X broadcast packets per minute and needs Y CPU time to process each broadcast packet. If you have 100 hosts on the network, there will be 100X broadcast packets per minute and each of the 100 computers will need Y CPU time to process each one, for a total cost of (100X)(100)(Y) or 10,000XY.

Now imagine we put those 100 hosts on 10 networks of 10 computers each. We have 10 networks, each with 10X packets and each packet causing each of those 10 hosts to use Y units of CPU. So the total CPU used by broadcast packets on those 100 computers is (10)(10X)(10)Y or 1,000XY. So by splitting into 10 networks instead of 1, the amount of CPU consumed by processing broadcast packets has gone down by a factor of 10.

because 1 network is limited. Imagine getting all 254 people to download at once, it would take longer than a girl's period.

Spreading out networks allows all 62 hosts to have faster connections. Mathematically speaking, 3.96 times faster than 254 hosts per net.

Functional delineation. As an example, why would you want to put your office servers in the same subnet as your desktop PCs? And going even further, you can have each department in their own subnet, with one or more routers in between firewalling traffic as appropriate.

couple of reasons

Most commmonly its used for management purposes, segment the network in to multiple pieces and group like thing together. Also its easier to secure an entire network and indivual hosts

Examples, group servers together in one network and users in another, you can then control access to the servers at the router.

Broadcast domains, computers use broadcasts to work out where other computers are (ARP requests) when you get alot of computers all talking to each other, this can be quite alot of noise. ARP requests (broadcasts) dont cross router boundries so its a good way to keep things isolated

And to the the guy above who suggested it was faster, no, no, just no.

security reasons... they may want one department to be segregated from another... say one department cannot access the other network but the other network can access this network...