The term "Molecular Biology" is very large in scope and covers a vast array of scientific professions (sometimes referred to as sub-disciplines). This includes, but is not limited to: microbiology, cell biology, immunology, virology, genetics and to a lesser degree even bioinformatics. So, it would be difficult to specifically address what it is that Molecular Biologists (often simply called a "researcher") do. That being said, all of these sub-disciplines are hypothesis driven. Meaning, that the molecular biologist develops a hypothesis (usually be building off the work of a previous researcher and/or through extensive reading) that he or she would like to test. They then design a series of experiments to "test" that hypothesis. Sometimes experiments can be quite complicated and require the researcher to develop new techniques. So, from the perspective of designing experiments, molecular biology requires a lot of creativity. It also requires a substantial amount of problem solving skills, because once a researcher has conducted their experiments, they must be able to read and interpret the data those experiments generated. Sometimes, this can be more complicated that designing the original experiment. And it often requires the researcher to design new experiments to confirm the finding of the original experiment. It may sound a bit confusing, but it's actually quite eloquent.
It's typically the excitement and challenge of designing experiences and collecting and interpreting data that attract people to molecular biology. Sometimes individuals have a personal connection to their discipline. They may have had a relative that had cancer or simply have an interest in "bugs" (a generic term used by some microbiologists to refer to all bacteria). For me personally, I've always enjoyed science and likes to be challenged intellectually. I was particularly inspired by the book "Shadows of Forgotten Ancestors" by Carl Sagan, one of the greatest science writers that has ever lived.
Regarding our impact on the world, you'll get very different opinions on the matter. Personally, I'm a bit of cynic. I feel that Molecular Biology is one of the most self-serving professions. I say that because the primary purpose of molecular biology to to conduct more molecular biology. I would estimate that 99% of molecular biology experiments simply lead to additional molecular biology experiments. Of the remaining 1% of molecular biology experiments, the vast majority lead to advancements in techniques or equipment used to conduct molecular biology experiments. It's the remainder that leads to advancements that benefit humanity. While that my be a pessimistic view of Molecular Biology, I believe it to be accurate. It take a very large investment (in time and money) to yield the few advances that benefit humanity, so without the 99% of molecular biology that doesn't produce any advancements, you would have that small part of the 1% that improves our lives.
And something that most people might not no about molecular biologists, is that we're not all as smart as we think we are.