Q: connected to wifi but cannot use internet

Does the iOS device connect to other networks? If yes that tends to indicate a problem with your network.

Does the iOS device see the network?

Any error messages?

Do other devices now connect?

Did the iOS device connect before?

Try the following to rule out a software problem:                

- Reset the iOS device. Nothing will be lost

Reset iOS device: Hold down the On/Off button and the Home button at the same time for at

least ten seconds, until the Apple logo appears.

- Power off and then back on your router

.- Reset network settings: Settings>General>Reset>Reset Network Settings

- iOS: Troubleshooting Wi-Fi networks and connections

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

- Wi-Fi: Unable to connect to an 802.11n Wi-Fi network      

- iOS: Recommended settings for Wi-Fi routers and access points

- Restore from backup. See:

iOS: How to back up

- Restore to factory settings/new iOS device.

If still problem and it does not connect to any networks make an appointment at the Genius Bar of an Apple store since it appears you have a hardware problem.

Apple Retail Store - Genius Bar

You also asked "why should i reset it in order for wifi to work and what should i do to fix this problem? " Because sometimes there is a software glitch causing the problem and restore, or even a restore is required sometimes.