Biology is extremely influential
It is easy for a science and tech savy person to overweight the technological impact of, say, computer science, and down weight the social impact of biology.
The science book often considered as the most influential of all times is On the origin of species (see TheGuardian for example).
Among the most influential science books of the past 50 or so years many are popular biology books. Consider this website for example. Among the 10 most influential science books, 6 of them are directly about biology, 2 of them are about Environmentalism but written by biologists, one is about math and one is about physics.
As a consequence, Biology.SE receives a lot of lay questions.
Biology is not as scary as physics is!
A physicist meeting someone in the street:
- I am a physicist
- Wow... you must be so smart
A biologist meeting someone in the street:
- I am a biologist
- Oh.... I like birds too!
By personal experience, it feels true to me that biologists tend to have less knowledge in math and quantitative sciences than physicists (particularly obvious among first year Bachelor degree students). This is almost paradoxical as much of modern statistics has been invented by biologists. Many chemists I know seemed to be pretty bad programmer and statisticians though (no offense to chemists).
The point I am trying to make is that biology tend to scare less. This is great when the goal is to educate a large audience. It can be an issue as some people may assume that they know best because they watched a 10 minutes BBC emission.
Evolution for example, is typically one of these subjects where laymen may quickly assume they know everything there is to know. It has happen to me a few times to face OPs that assume they know a whole lot about evolution while they don't. Of course, the first step in learning is often realizing what we don't understand, so it makes everything more complicated.
"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so." Mark Twain
Religious extremists are often very opinion-based when it comes to science. Of course, many creationists will fail to accept some elements of physics such as the big bang theory but I think that most of deep physics is far enough from many people everyday life and intuition that the clash is less important than with evolutionary biology. Not to forget that the modern evolutionary synthesis makes a great deal at explaining the selection pressures behind behaviour which obviously deeply touches people in their own identity and may create clashes with an essentialist philosophy.
On Biology.SE, we even have the chance of having an anti-evolution spammer who asked about 10 times the same question about nylonase! Btw, I just realized it is a common "creationist argument" (see Nylon-eating bacteria and creationism)
Biology has a big impact on the economy and on ethics
I think that biology is particularly prone to yield some laymen to confound ethics and politics within their post. The typical subject on which opinion-based question come up are about GMO.
Of course, other fields of science have a big impact on the economy.
Fuzzy definition of biology
Of course, no science is perfectly, non arbitrarily defined. There is a fuzzy boundary between math and computer science, between chemistry and biology, between geology and physics, between chemistry and physics, and probably between pretty much any two fields of science.
Biology is the science of life. This may make it feel to some that any subject of medicine, psychology, archeology and civilisation studies, veterinary sciences, agriculture, pest control, gardening and climate science would fall within this category.