Media Access Control Address aka MAC Address plays a vital role to connect to the internet. To use internet, you must have network card and every network card has one unique MAC Address. If you have any MAC Address related issue, you may fail to use internet.
If you are using Wi-Fi router to use internet from all devices at your home and Wi-Fi router has a weak password, your neighbor can easily steal your data. You will certainly get very low speed if 2-3 devices use your router simultaneously. In simple words, you need to secure your Wi-Firouter using strong password. When it comes with Wi-Fi security, MAC filtering comes first.