Compelling reasons to have a coin professionally graded are, first and foremost, based on value. That value could be monetary due to grade/rarity, or perceived due to demand. Secondarily, one would wish to submit a coin if authenticity is a question, but the first reason applies here also.
In all but one eventuality, this process is meant for those who follow you, not yourself. That one exception is if it's your wish to participate in the various
TPG Registries. Not for me, but a perfectly valid reason for collectors who wish to swim in those waters.
Otherwise, you have a coin graded against the moment when it leaves your collection. Perhaps you wish to liquidate it in the near future; perhaps you are protecting the interests of those who will inherit the collection upon your passing. Graded coins are far easier for an heir who may not have numismatic knowledge, and many collectors have the specific interest of providing economic gain for their heirs via the collection.
When authenticity is in question, plenty of resources are available beyond the
TPG route. Dealers and individual specialists who can conclusively evaluate your coin abound; in many cases you can simply post photos of appropriate quality at a place like Coin Community.
I mention "perceived" value. Just because a given coin isn't worth a lot today, does not mean it won't increase in value tomorrow. Varieties become recognized for rarity unknown in the present; collectors turn to new series either through success or inability to succeed against demand in their current series. I see a particularly strong future upside in my own concentration of
Morgan dollar varieties.
....which is the specific context of my comment regarding "cherrypicking." In the current state of the specialty, collectors of
Morgan dollar varieties are prone to offer more enthusiastic money for coins they perceive to be unknown to the seller. It's a purely psychological effect, unrelated to the true value of the variety. Some varieties are of known rarity and value; these are better bought and sold in an attributed slab.
It all boils down to value, though. For me, that arbitrary value line is in the vicinity of $150-200. Today, or in the future.