What does Clear App Data do on my phone?

rossross

I have a 5 years old Acer Liquid Zest T06 Android 6 with 1GB RAM, 8GB ROM and a 30GB SD card added and recently I'm getting low internal storage space warnings. Except Viber and Skype which are moved on the SD card, I have not installed any additional apps but just upgrades to the originally installed apps that I use. I have set the photos taken to be backed up on Google Photos cloud and do not keep them on the phone. The Gallery items are on the SD card. I don't know what keeps taking the internal storage space, maybe the updates inflated the apps space but I'm wondering what can I do to free up storage space. When I use the System Doctor app and go to Apps > Data it shows a list sorted by apps data size with an option to clear it. On the top of the list taking large app data space are Gmail 481MB, Messaging 163MB, Drive 140MB, Google Text-to-speech 117MB. The rest of the apps range from 20MB down to 12KB. I wonder what will happen if I clear the app data of the mentioned apps and if that will help. I searched online and spent a lot of time reading various info but didn't find it very helpful.

I will greatly appreciate your help and any input

rossross

Forgot to mention that Google Play Store also had more than 100MB app data that I cleared to see what happens because I don't see what I can lose there and now its app data space is only 11.12MB. I opened it and didn't find any difference from before clearing its more than 100MB app data and I'm wondering what were all those extra MB used for.