Venture capital is typically used to either:
Build out or prototype something. This may be something inexpensive but for some reason can not be bootstrapped off of customers (e.g. a new SaaS product) or is something capital intensive that may have a giant market on the other side. The later includes things like building rockets for spaceX, or biotech drugs.
Scale something that is working. For example, you want to add sales or go-to-market functions to sell faster/better, or your consumer app is growing like crazy and you want to be able to add more compute to serve users. Uber needed to raise billions to both scale rapidly and beat out global competition.