It really depends on your trip.
Popular hotels and restaurants in Praslin offer Wi-Fi services. However, they are sparse. Access is limited to public spaces like the swimming pool, buffet area, and so on. So, if you leave those areas or the hotel, you will find yourself without internet.
If you need internet when you are on the move, say for using internet-enabled GPS on your phone, you need a portable Wi-Fi hotspot.
In short, if you wish to enjoy the beauty of Seychelles thoroughly and still want to stay connected throughout your experience, then you need portable internet in Seychelles.