In English, one letter may have many different pronunciations. If you hear a word, there is no way to know, just from the sound of the word, how to spell it. However, a written word will frequently have only one way you can say it. The words "goatee" and "ghoti" for instance would both be pronounced the same way, and even though "ghoti" is not actually a real English word, we know how it sounds just from how it is written. (see: http://zompist.com/spell.html for more details on this)
The ability to spell the same SOUND in many different ways is useful, because there are many words that sound the same, but have different meanings. "There" and "Their" is one famous example. Even though these sound exactly alike, it is useful to have different spellings for them, because the meaning is very different.
Spellings can also give you information about the "root" of the word, or its origin, or even its history. English borrows words from other languages very often, and we typically like to keep the original spelling from the original language, even though our spelling rules may be different. Over time, the way people say words can also change, but we usually keep the spelling the same. (For example, the word "Knight" used to be pronounced "Kh'ni'git", which is more or less how it is spelled, but when we changed how the word sounded, we did not change how to spell it)
Spelling can also inform you about the grammatical function of the word. To use your example, "Titanic", this word comes from the word "Titan" and the suffix "-ic", which makes the noun into an adjective. This "-ic" ending, however, can only be used with words that come from certain languages, specifically Latin or Greek. So by seeing that the word ends in "-ic", we know that the word comes from Latin or Greek, and is an adjective. Knowing that the word comes from Latin or Greek is important because the origin of the word tells us a lot about how formal the word is. For instance, Latin or Greek words are much more formal than German words.
Sometimes there is no real reason for one letter to be used over another, but once a letter was picked, it just stays that way forever.