there are no rules for that. tongue position for soft consonants is easier in junctions with certain sounds, therefore due to the law of natural laziness foreign words get soft signs at times.

There are some rules for use "ь" in the Russian language. In the words as "компьютер", "ь" (soft character) is the separation character. We use it before "я" [ja], ё [jo], е [jэ], ю [ju], и [i] inside the words, but not after the prefix. For example: "серьёзный", "вьюга", "соловьиный". After the prefix we use separation character "ъ": "объект", "двухъярусный", "трансъевропейский". In the words "гольф", "фильм" - "ь" is literally the soft character: we use it to soften sounds in the words. This is peculiarity of the pronunciation of many words in the Russian language.

we have no rules when to ad "ь" or when not. You should remember all this words. We add "ь" for easier pronanciation of the consent

http://www.rusyaz.ru/pr/ob01.html Буква ь пишется внутри слова не после приставок для отделения в произношении согласной от следующих за нею и, е, ё, ю, я, например: карьер, вьюн, бурьян, подьячий, семья, ружьё, ночью, рожью, воробьиный, курьёзный, лисье, лисью, лисьи, чья, чьё, чью, пью, шью.

In case of компьютер, I guess "ь" is added to make Russian hard "п" sound into English soft 'p' sound.