Here are the 4 probable reasons why you really need a phone holder in your car.
If you want to reach home or at work safely, you will have to put all of your attention on the road to avoid possible accidents e.g hitting a car or a person when you are not looking and are on your phone instead.
A phone holder takes up the responsibility of holding your phone for you as you drive. You only need to anchor it in a position suitable for you to give it a quick look, maybe when you are using a GPS or picking a call. (Do not SMS when you are driving)
A cupholder phone holder is the most recommended phone mount as it securely holds your phone and stays solid on the cupholder.
Your phone hits the floor of your car, breaks the screen, internal electronics get distorted and your expensive phone gets irreparable damages. You would not want this to happen to your phone.
Having your phone anchored on a phone holder ensures that your phone is secure(no falling) even when off-roading.
Phone holders are also built to prevent phone scratches.
Handfree phone usage
You want a simple driving experience, then you need a phone holder in your car. Most online searches including GPS locations can be done using voice. Even answering and making a call can all be done by a simple voice command to your smartphone.
When driving, you do not need to constantly interact with your phone, or things may go south without notice.
With your phone being attached to a steady phone mount, you just need to make voice commands to your hands while all your attention is directed to the road ahead.
Comply with local regulations
Different countries have varying regulations when it comes to smartphone usage in your phone.
What we know is that most of these countries allow the usage of a phone in your car while driving only when you are using a phone holder.
Here is the link to the USA road safety regulations