Although some founders might pursue high valuation as a way to get validation, the majority or at least good founders pursue high valuations to avoid dilution.

Another big reason would for recruiting. I've seen that's become increasingly common that job offers include options as prices instead of percentages. Eg you'll get $500,000 in stock based on the last round valuation as opposed to saying 0.5%. This is increasingly common in later stage companies and it seems to be very effective. Now companies in early stage with high valuations can do the same.