(Real) investment, in the sense of building up new productive capacity, is an important part of growing the economy. However, investment at least by the private sector cannot thrive in a vacuum. It needs a context of either existing or plausible demand. If the demand is missing, you'd be a fool to spend money on increasing productive capacity, i.e. you'd be a fool to invest.

Note the important emphasis on what kind of investment we're talking about. Unfortunately, the act of buying existing productive capacity (e.g. by buying stocks) is also called investing, and you have to be careful not to confuse the different meanings of the word.