The confusion comes in when we conflate "salvation" with "ending up in Heaven for eternity."
"Salvation" has three meanings in Scripture:
Initial conversion (being "born again")
Sanctification (of believers)
Ultimate redemption (resurrection, etc.)
It is clear that the first two can happen without the person necessarily ending up in Heaven for eternity, given the many passages that clearly show that a person can "fall away". (Really, a case could even be made to include the last sense of the word as well based on passages like Matthew 22:1-14.)
Who ends up in Heaven for eternity? Only God knows, but we can trust that He is just, so we don't really need to worry about messing that up. Truth be told, I'm not even convinced a person has to be born for Him to make this decision justly. (See here for more.)
With that said, the value in praying for someone to "get saved" or "continue to experience salvation" or "be saved on the last day" is the same as the value in praying that an unbeliever would experience a clear sign from God, or begin to seek God, or recognize that God is trustworthy.
It is good to experience the kindness of the Lord. If you love them (which you should), you should do good to them and desire good for them.
You can join God in His efforts to draw His "lost sheep" to Himself. God's preferred method of blessing is often via the prayers of His people.
Even if you pray and they "get saved", but the person never ends up in Heaven for eternity, at the very least they will have no excuse on judgment day.
So, in summary, the value in praying for a person's salvation is the same as the value in praying for any good thing for another person; It is loving, it is partnership with God, and it is a witness to the "wicked".