Comment adverbials and set phrases for a higher speaking score.
In the IELTS speaking exam you will be judged on the range and accuracy of your vocabulary, so having a wide range of expressions obviously helps your score.
Comment adverbials are a good way to show emphasis that something is surprising, true, disappointing, coincidental, unbelievable, fortunate or unfortunate. These are mainly used in speaking. However, in IELTS writing they come across as being too informal.