For history, you are expected to have done lots of wider reading (you get provided with a reading list, well at least I did doing AQA), as this is used in arguments in the form of historian interpretations and allows concepts that are complicated to be understood through the lens of differing historians. Additionally, in lessons you are basically taught the outline spec, therefore wider reading is necessary to get to grips with the full picture.
In biology, wider reading in regards to the 25 mark essay is literally worth about 2-3 marks, they don't expect you to go off on one, but i think its like linking something in the spec to outside of the spec to answer the questions. I remember everyone complaining about this when doing biology but my teacher said no one hardly ever gets full marks on the essay, and as long as you cover the scientic breadth and actually answer the question you should be fine... The 25 markers may seem like they only apply to one topic, but trust me with more practice you'll realise you can talk about so much!