You already got the answer.
The Tabernacle with a lit (almost always with a red glass chimney) candle has the Reserved Eucharist. Since a Priest can only Confect the Eucharist during a Mass, if he gets called out to someone’s deathbed, he needs a supply at the ready.
But that’s not the main reason I had to reply. Show up at Mass Holy Thursday. It’s fast approaching. At the end of it there will be a procession with the Eucharist being carried out of the church. The Tabernacle will be left empty until Easter.