Her name means "heaven" in Latin, and after Redcrosse faces Despair and almost dies, she's exactly what he needs: someone embodying virtue and the importance of a life balanced between praying and doing good deeds. She's pretty heavenly.
You could kind of think of her house as a spiritual spa, and it's appropriately known as the House of Holiness. Reflecting I Corinthians 13:13, that identifies Faith, Hope, and Charity as the three most important virtues, Caelia has three daughters: Fidenza (close to the Latin for "Faith"), Speranza (Italian for "Hope"), and Charissa (close to the Latin for "Charity").
Redcrosse meets with her daughters in the order of how they should be experienced in life; first you need faith, then you can find hope, and after that you are prepared for charity. Charity is specifically described has having just had a child, which may represent the fact that Redcrosse has recently been reborn into faith after his devastating encounter with Despair.