Closing the country, or at the very least restricting access had several perceived advantages for the various rulers of the Tokugawa-Shogunate. On a domestic level, it prevented foreign powers from influencing the various feudal lords, which may have risen up if they managed to gain foreign support. Clans like the Shimazu or Mori had resisted the establishment of the Shogunate and had most of their lands on either Kyushu, or the western part of Honshu and were therefore more likely to be engaged in trade and diplomacy with either China or European countries. I don't believe it is fair to suggest that the sakoku policy is the only thing, which allowed the survival of the Tokugawa-Shogunate into the 19th century, but is telling that only 15 years after the country was opened, forces from Kyushu(among others) were instrumental in the overthrow of the Tokugawa.

Restricting trade with European countries to the bay of Nagasaki, which happened to be in the personal domain of the Shogun meant that he had a valuable trade monopoly. It should be noted however, that trade with Ryukyu, the Ainu on Hokkaido and Korea still occured during the entire duration of the sakoku policy, although mainly through the local feudal lords, rather than the Shogun, himself, meaning that he never possesed an absolute monopoly on foreign trade.

The last point would be Japan's position in the international order of East Asia. The Ashikaga-Shogunate had accepted its role as a tributary of China. Toyotomi Hideyoshi, after overthrowing aforementioned Shogunate, invaded Korea, a fellow tributary. This was both a breach of the customs which discouraged such wars and probably more important in the grander scheme of things, potentially contributed to the increasing power of the tribes of Manchuria, that would later establish their own Chinese dynasty, the Qing. After Hideyoshi died, his successors had no interest in conquering Korea and attempted to improve the relation of the two countries. They were however not willing to once again accept China as a superior power and didn't establish diplomatic relations. Instead, both Ryukyu, Hokkaido and to a lesser extent the Dutch, were treated as tributaries themselves. The sakoku policy can therefore be seen as a way to 'free' Japan from its role as a Chinese tributary, by essentially ignoring China and attempting to build up a similar international order around Japan itself.

To sum it up, the perceived benefits were domestic stability, a steady source of income, while preventing lords from profiting off the trade with European nations and sovereignity in foreign affairs.

If anyone wants to read a bit more, these articles are available for free on jstor.org:

Totman, Conrad. “From Sakoku to Kaikoku. The Transformation of Foreign-Policy Attitudes, 1853-1868.” Monumenta Nipponica, vol. 35, no. 1, 1980, pp. 1–19. JSTOR, www.jstor.org/stable/2384397.

Kazui, Tashiro, and Susan Downing Videen. “Foreign Relations during the Edo Period: Sakoku Reexamined.” Journal of Japanese Studies, vol. 8, no. 2, 1982, pp. 283–306. JSTOR, www.jstor.org/stable/132341.

Not for free(as far as I know), but still helpful:

Laver, Michael S.: The Sakoku Edicts and the Politics of Tokugawa Hegemony. Amhurst. 2011

Cullen, Louis Michael: A History of Japan 1582-1941. Internal and External Worlds. Cambridge. 2003.