Philae Temple, dedicated to the goddess Isis, originally stood on a different island that was gradually submerged as dam projects on the Nile raised water levels through the 20th century. The entire complex was dismantled and reassembled stone by stone on nearby Agilkia Island, safely above the waterline, as part of a well-documented international engineering effort.
The temple is part of the "Nubian Monuments from Abu Simbel to Philae" UNESCO World Heritage site, inscribed in 1979 — one of only 7 UNESCO sites in Egypt overall. It's open daily 7:00am–4:00pm; general admission for foreign visitors is EGP 550 for adults, EGP 275 for students (Egyptian nationals: EGP 40 / EGP 20), with a separate, cheaper "temple panorama" ticket also available.
Why it's a near-universal stop
That relocation history, combined with the temple's setting — reached by a short boat ride across the water — is why it's included in nearly every Aswan itinerary, cruise-based or not. Visits are typically timed for the morning or offered as a standalone excursion by boat; an evening sound-and-light show is also commonly available.
For the other major site most Nile cruise itineraries build a full morning around, see Is the Valley of the Kings Worth Visiting?