#1: The frequent use of loan words in Japanese is probably one of the most helpful aspects of the language for foreigners. The following picture contains a sign at self service gas station in Japan, and it can easily be read by English speakers as long as they are familiar with all the katakana symbols in Japanese. This is because English speakers can typically sound out Katakana symbols phonetically and connect them to the English words from which they were originally borrowed. (The method does not always work because Katakana words are borrowed from other foreign languages as well).
The following words are taken from the sign in the picture above.
レギュラー : regular
セルフ: self (service)
ハイオク : high octane
#2: Katakana can also be used for sound expressions. In the following example, you see the representations of the sound the dog makes when sleeping: Guuu, Guuu, Guuu... (グウー)
#3: Katakana words can also be used to replace very complex, and "difficult-to-write" kanji, such as very technical terminology in medicine. 皮膚科 (dermatology) is typically written as ヒフカ.