Почему после appears ставится to be, всегда ли надо его ставить?

Потому что это чужой язык и в нем свои правила.
Если вам действительно интересно, когда и что "ставится" после appear, имеет смысл заглянуть в хороший словарь. Там вы найдете примерно семь значений этого глагола, из них два - как глагола связки, из этих двух - семь моделей употребления в первом случае и четыре во втором. Например:

1) V-LINK: (vagueness)
[there V ] There appears to be increasing support for the leadership to take a more aggressive stance... [V to inf] The aircraft appears to have crashed near Katmandu...
[it V that] It appears that some missiles have been moved...
[it V that/to-inf] It appears unlikely that the UN would consider making such a move...
[V n] The presidency is beginning to appear a political irrelevance...
[V adj] Nine months later, those talks appear as distant as ever...
[V adj] He appeared willing to reach an agreement.

Изучение языка предполагает самостоятельную работу со словарем. И поверьте, тем единственным значением слова, которое вы знаете, дело не ограничивается.

to be - когда что-то кажется (быть) каким-то. Вот несколько примеров без to be: If I did not know better it would appear that you are causing this conclave intentional danger." As soon as signs of decomposition had begun to appear, the whole aspect of the monks betrayed their secret motives in entering the cell. However, generalizations and slight variants of the problem appear very often in a large number of diverse application areas. When corporate training companies began to appear in Russia some 10 years ago, not many people knew what the industry was all about. Is it not time to cease terrifying society with the appearance of the “red spectre” from a direction from which the red banner of the working class can never appear?