There is a small amount of pressure on the hydraulic lifter plunger holding the top retainer clip in place on a dry lifter.
It can be pushed down with your finger to install the clip.
But with a lifter half or more full of oil, it's harder to work the plunger with your fingers as the pressure behind the plunger is greater.
If you install the lifter into a holder, that frees up both hands and gives better leverage on the plunger.
The holder below is fabbed from a steel sheet and a welded bushing to hold the lifter.
A 2×4 block of wood with a 1“ hole drilled into it can also be used to hold the lifter.

Below is a fabbed steel holder.
The block is used to press the plunger down. A spare pushrod or screwdriver can also be used to depress the plunger.

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