It's pretty useful if you are driving a car and don't want to be disrupted or if you are busy doing something else that doesn't allow you to pick up the mobile phone in order to read the message.
As soon as the app is installed it will be notified about incoming SMS and start the synthesizer. It tries to look up the incoming number in your phone book and will tell you the name of the person unless you disabled this in the settings.
Additionally you can specify a silent time when your phone should not read messages e.g. at night.
Several smileys like :-), B-), XD or :-D will be proncounced as giggling, chuckling or a laugher. The :-* smiley will cause a kissing like sound. If you know that the recipient of your message also uses the SMS Reader app, you can suppress reading the message out loud by prepending two dashes to it such as '--(This text will not be read)'.
The app does not request permission to access the internet, hence you are safe that none of your personal data is transmitted anywhere. It only needs permission to receive short messages and to read your contacts to look up the display name.