Sooner or later you may wish to hear your Raspberry Pi speaking.
It's nice to get messages from your RPi by Whatsapp, or reading on a small display (or TV), but hearing this device answering you with a real voice could be amazing!
There are several TTS (Text To Speech) engines avaliable for a Raspberry Pi and you can find a list here.
Usually offline engines suffer from low quality voices (and sometimes limited languages avaliable). On the opposite side, online TTS engines could have really high speech quality, but you need an internet connection (and your recorded voice will be analyzed from someone else).