Imagining what you want to be is how you become what you are.  A colleague lamented to me, “if only it was that easy”.  Arguably, it depends on the interpretation.

Reasonability

If you imagine being a prince or princess with great wealth and respect, in a literal sense, there is a high risk you will not become that. Figuratively speaking, maybe you can.

Imagining things that are in the realm of the possible (reasonable expectations, albeit they may be stretch goals) is a technique which supports the achievement of the goal.

Imagining the end point does not need to be exact.  For example if you imagine yourself a world class athlete, stepping up to the podium for your gold medal, taken-in the figurative sense, that is to be a successful athlete, your imagination can be a valuable guide.

Seeing, Acting, Achieving

Visualizing your success, even if an exaggerated version of it, helps you believe it is possible.  Confidence based on belief begins in your brain.

The caution of course is not to become overconfident,  but rather leverage the imagination as a guide, not as an absolute.  Striving for excellence in your mind is a beginning, ultimately though you do need to focus on the possible.

Imagine what you want to be, then become what you imagine within the constrants of reality.  This is not dreaming, rather it is a practicle approach to goal setting.