A home "router" such as the Netgear N600 is actually an integrated router that also includes a 4 port gigabit switch, and a wireless transmitter with this particular model transmitting 'up to' 300mbs over the air. There are also additional features included such as a firewall, QOS, etc. The definitions of router vs switch usually refer to enterprise routers and switches, NOT home 'routers', this misunderstanding is common.
If you go to Best Buy when you ask for a router they will show you what are actually
integrated routers that usually have a 4 port switch included, this is where the definitions from router vs switch on a Google search get confusing because an enterprise router is much different. If you ask for a switch while at Best Buy, they'll show you to some lower end switches that would generally be good for a small business. These will have more ports available, however they won't have any of the features available in an integrated router, and the main feature you'd be loosing is DHCP and NAT which allows multiple PC's to automatically acquire an IP address and access the internet.
The next thing I want to clarify is regarding to how gigabit works. This is
ONLY available using one of the 4 ports on the N600, you cannot get gigabit speeds connecting to it wirelessly. Keep in mind you will only have gigabit speeds in your local network - in other words - you can only transfer files at gigabit speeds between devices in your house that also have gigabit ports, none of this has any effect on your internet speeds. If your PC says it is connecting at 100mbs then your PC probably only has a 100mb NIC card. You could upgrade, but unless you have a home server from which you stream movies from this wouldn't help improve your internet speeds one bit.
If you connect wirelessly you can connect 'up to' 300mbs with the Netgear N600, but various factors can effect this, and just like with gigabit, you need a wireless NIC card that supports 300mbs.
With all that said I highly recommend against using both a 'router' and a switch together, (by which I mean an integrated router found at Best Buy or similar stores). Unless you need more than 4 wired ethernet ports you don't need an additional switch. If you are connecting wirelessly to the N600 this is probably the reason for fluctuations in your speed. If your PC supports it, I would connect via "N" as opposed to "G". Distance from the router as well as thickness of your walls will also play a role connection speeds. I would test your speed both from your wireless connected PC and also a PC connected with an ethernet cable to rule out the speed fluctuations being attributed to your service provider.
Feel free to ask any further questions, the advertising jargon that Best Buy nimrods repeat usually just ends up in people paying too much for a home router that has features they can't even use in their current setup. I'd gladly help someone avoid paying too much at Best Buy - I guess I consider this my payback for them not hiring me when I was a teenager because I couldn't sell the boss a stapler during the interview
So with all that said, to answer your question: Gigabit switches increase the throughput (speed) of a local network. Most home users don't need a gigabit switch because most people don't do much more than simply access the internet. Home routers with gigabit ports are mostly just a marketing gimmick to convince people to shell out money for something they don't understand. However, gigabit switches are popular in businesses because employees are connecting to local servers, and in some cases doing voice over IP, etc. For those purposes a gigabit switch makes perfect sense, but at my home I could care less.