The Nephites were taught by Nephi that "ye must bow down before him, and worship him with all your might, mind, and strength, and your whole soul." Where did Nephi’s people worship? In the temple! Soon after Nephi and his people settled in the land they eventually called "Nephi," they built a temple.
Because Nephi’s people were still living the Law of Moses, they needed a temple to obey many parts of that law. He instructed his people: "[I]nasmuch as it shall be expedient, ye must keep the performances and ordinances of God until the law shall be fulfilled which was given unto Moses" (25:30). Among other things, the Law of Moses was the Law of Sacrifice. Nephi’s temple had an altar. The people offered appropriate sacrifices on certain days, as specified under the law. They would have also observed certain festival days such as the Day of Atonement or the Feast of Tabernacles, which were connected particularly with particular sacrifices and solemn observances.
However, Nephi was well aware and knew that all ordinances and covenants under the Law of Moses pointed to the coming of Christ. Nephi preached of Christ and also the importance of fulfilling the requirements under the Law of Moses. The word "expedient" in verse 30 probably means "appropriate" or "necessary." Nephi was explaining to his people that it was still necessary for his people to perform the ordinances under the law of Moses because that law had not been fulfilled. He also understood that, with the coming of Christ, the performances and ordinances under the Law of Moses would be fulfilled.
Book of Mormon Central, "Did Ancient Israelites Build Temples Outside of Jerusalem? (2 Nephi 5:16)," KnoWhy 31 (February 11, 2016).
Book of Mormon Central, "Why Does the Book of Mormon Say We Must Worship God with Our Whole Soul? (2 Nephi 25:29)," KnoWhy 467 (September 13, 2018).