In situ conservation doesn't share the burden of storing the genetic material so it is much cheaper. This century we lost at least a thousand species a year. That means that one out of every eight plant species is at risk to go extinct. We aren't actually hurting the species directly but we are damaging and often killing the ecosystems in which plants need to grow. Plant species need biodiversity to continue adapting to the environment in which they live.
Biodiversity is lost when humans take over what is grown, such as crops.
It is also lost when gene pools within species evaporate and they have no wild ancestors. In the U.S. alone, two thirds of all rare and endangered plants are close relatives of cultivated species. That means that the species that are going extinct aren't just some bush or weed that we don't need. They are actually the wild ancestors that enable biodiversity to grow. The fact of the matter is that we seem to be in a race to keep these wild ancestors in such places as botanical gardens and gene banks.