Specific, not general
Glossaries are absolutely vital when it comes to preparing to speak or write on a particular subject.
They are different to your normal vocabulary lists and notebooks because they are basically a collection of useful vocabulary on a specific topic.
At A Level stage (or other advanced language learning stages), it is not just enough to learn good general vocabulary, such as sentence starters, conjunction phrases, and phrases with complex tenses, you also need to know topic-specific vocabulary in order to speak confidently and authoritatively on any subject.
Produce, not just understand
When you read or listen to something on a subject area, you may well be able to understand most of what is being said, but that is a different skill to being able to produce that vocabulary when it is your turn to write or speak on the same subject. You need to have key, useful words and phrases on that particular topic right in your memory so that you can use them and prove your language capabilities.
Wide research, not limited
Compiling a topic-specific glossary involves a lot of research. This means reading and listening to a wide variety of texts on the subject so that you can note down the phrases that you could recycle in your own way. The more research you do, the more vocabulary you will have and the more you will be able to hone it down to the words you want to focus on learning.
Always keep your glossaries and be continually adding to them as you discover more and more key words and phrases. This is a skill that you can keep for the rest of your language-learning experience.
A helpful starting point
Check out my advanced learner glossaries, packed full of key vocabulary on A Level/Advanced Higher topics, easily downloadable a great place for you to start building your topic-specific vocabulary.