This is good idea to get the Wi-Fi Connected, only when needed. By switching on the Wi-Fi, whenever needed, may not only save unnecessary Internet usage but also a lessened battery consumption. That’s why, Android is always careful to add features to switch on the Wi-Fi, whenever needed. So, Android has also added feature which can switch on the Wi-Fi, very easily, whenever needed.
You may switch on the Wi-Fi in a few steps but if you feel need, just see the instructions.
Instructions
1: Tap the status bar on the Home screen of your Android 5.0.2 Lollipop device to open up the ‘Notification bar’.
2: Tap the ‘Notification bar’ and drag it downward.
3: See the ‘Wi-Fi’ is not switched ON. Tap it and hold for a while. It will take you to the ‘Wi-Fi’ screen where the status of the ‘Wi-Fi’ is ‘Off’ with the color of the switch is Grey.
4: Tap the ‘Wi-Fi’ switch and drag it to the right to switch ON. See also the status of the Switch is changed to ‘On’ with the color of the switch is also changed to Green, indicating that the ‘Wi-Fi’ is switched ‘On’. 
That’s all!
All you have to do is tap once on the Wi-Fi icon on the quick settings panel and it toggles Wi-Fi on or off. There’s no need to hold down on it and go into the Wi-Fi settings to toggle it. In fact, you don’t even have to hold down on the icon to go into the Wi-Fi settings. You just tap once on the word “Wi-Fi” under the icon.