Its a loaded term, with lots of significance to most, depending on perspective.
For the biological realm in general, it is very important. Why?
Initially, the first living things here on Earth using DNA or RNA reproduced asexually. Meaning, they simply made copies of themselves. That's fine so long as you are in environments where the same old thing is "good enough."
But in a world with environments that change with time or location for any number of reasons, the same old thing isn't good enough. In fact, unless an organism can adapt, changing environments pretty much means extinction.
The way living things initially coped with change was via mutation, and it was rather random. The genetic material isn't perfectly copied every time, and the inevitable copying errors created differing properties or traits for the early life forms that possessed them. If they helped the organism thrive (meaning reproduce, grow, stablize, and/or defend itself better) then those new organisms were able to outcompete their weaker brethren with weaker traits. If the changes instead were detrimental (and, they often were), then the stronger brethren would survive better than those with the new mutations.
However, inheritance is generally best when more stable rather than less. Unstable inheritance leads to problems if it is too high. Afterall, when you have a recipe that works, you don't want to change it too much, all things considered. That is what inheritance is for, afterall.
Still, there would be an advantage for a species, or rather, a lineage, if it could take advantage of differences in traits that are known to work, rather than rely solely on random mutations that could be lethal as often as they were helpful. That can be achieved if organisms can exchange with each other their genes and variations of genes that are known to work. Exchange of genetic material between to individuals for the purpose of reproduction is termed "sexual." Thus, sexual methods of genetic exchange developed.
At first, sexual reproduction would have been much more basic than it is for humans today. Put simply, it would simply have been a matter of living organisms putting forth a complement of their DNA in a new cell (i.e. a gamete), which would then be joined with a complement of DNA from the gamete of a second organism (usually similar). Then the paired complements of genetic material would go forth and develop as a new individual, a hybrid of the two contributions. Contact between the individuals participating in this sexually reproductive process would not necessarily have been required. All that really would have been required was a way to protect the complementary genetic material so that it would not be damaged or destroyed before fertilization took place. And so you eventually had the production of specialized cells suited to the task - eggs, pollen, sperm, etc., which would be released into the environment and mixed.
You can imagine that leaving successful reproduction to change like that might be fine in many circumstances, but perhaps not fine in others. For example, if the environment is harsh, or if there is danger of the specialized gametes being destroyed or eaten, then a more certain method of delivering the sex cells to each other might enhance survival of the lineage.
Well, intercourse can help with that! And so the question arises ... just when did the mechanism of intercourse as a means of delivering gametes to each other for fertilization arise?
The video above offers a suggestion.
How wonderful that evolving life forms had the opportunity to try something different.