Do I need a router if I already have a modem with WiFI capabilities?

[LessThanSatisfactory]

I already have a modem at home, provided my my ISP and I get enough LAN ports and it comes with wifi capabilities. Do I need a router, and why? What does a router actually do, when it converts WAN to LAN?

[extremescouter]

A router is normally used to convert the modem's ethernet out into wifi and more gigabit ethernet ports. It's something like a combination of a gigabit switch, and a wifi hotspot. I think the only benefit of using a router for you would be the added security options, but in your situation, I don't think you need a router, as your modem is acting like a router. 

[EricX2]

Your modem appears to be a router. A router is a device that connects one network to another, typically your internal network to the internet. So no, a router wouldn't be needed unless you wanted more features such as 802.11 AC.

[LessThanSatisfactory]

Your modem appears to be a router. A router is a device that connects one network to another, typically your internal network to the internet. So no, a router wouldn't be needed unless you wanted more features such as 802.11 AC.

Is 802.11 AC compatible with all my previous wireless N gear?

[A32m]

To sum up, your ISP has already given you a router, but it is a modem+router device. An extra router would only add another lan pool network,

 

An example would be your modem/router LAN 1 port is occupied by your main device via ethernet.
Your LAN 2 port would connect to your new added router, and would create another network for another sub-main computer in the house, so that would mean 2 wireless and wired networks in your house.
This is a plus if another user in the home wants a private wireless and wired network that only the LAN computers connected to the second router can monitor.

But be mindful that bandwidth decrease with the more device/networks are added to the original router.

What is a router, and what does it do?
A router is a device that routes paths for the connected devices, to a WAN connection using routing protocols and such. A router generally contains switch-like properties.

In your scenario, it is not needed, because your ISP has already given you the wireless/wired network capabilities of a router in a modem. It is only needed for people who want to sub-divied their network for what ever reason. But in most cases, it is used to provide switch and wireless capabilities to a stand-alone modem (with only one LAN port).

Hopefully I answered your question :).

[Sfobrien52]

Just a tip, adding wireless devices (like routers) will decrease the strength of all wireless networks in the area