Motivation increases the closer we get to the goal.
A stamp card with 8 of 10 boxes filled motivates far more than one you've just started. We put in more effort, and faster effort, the closer we feel we are to completing a goal, even if the actual remaining effort is the same.
A study with a coffee shop's loyalty cards compared two versions. One required 8 purchases to earn a free item. The other required 10 purchases, but came with 2 boxes already stamped as a welcome gift, meaning the same real effort of 8 purchases. Customers with the 10-box card (2 already filled) completed their card faster than customers with the empty 8-box card, even though both groups needed exactly the same number of additional purchases.
We don't evaluate the remaining effort in absolute terms, we evaluate it as a proportion of how far we've already come. Feeling like we've already covered part of the distance, even if it's artificial, increases our motivation to finish it.
Source: Kivetz, R., Urminsky, O. & Zheng, Y. (2006). The Goal-Gradient Hypothesis Resurrected: Purchase Acceleration, Illusionary Goal Progress, and Customer Retention. Journal of Marketing Research, 43(1), 39-58.
A progress bar that starts at "20% complete" instead of "0%", or an onboarding checklist with the first step already checked off, take advantage of this effect without requiring the user to do anything extra at the start.
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