An investor can raise the valuation by putting in more money for the same ownership percentage or same money for less percentage.
They wouldn't generally want to boost valuation for their round because that reduces their return. But there is probably some wisdom in hyping up valuations to get customers and future potential investors excited.
Additionally, if it is a follow-up round, they probably want to invest at a higher valuation just for their own investor confidence. No one wants to have a "down round" because it throws cold water on the hype train for all the other investors.