The addition of the '-겠' generally adds a sense of the speaker's conjecture or opinion. It can be used sarcastically, as noted above, but it doesn't need to mean that. It depends on the context in which it's used. As to why '돕다' is '도와' and not '도워', it's because the committee that set the standard decided that 'ㅂ' irregular verbs with monosyllabic roots whose vowel is '아' or '오' have the stems change to '와' instead of '워'. Another example would be '곱다' -> '고와'. This is consistent with how the word would actually be pronounced. Prior to 1988, it was actually '와' for all 'ㅂ' irregular verbs whose stem had '아' or '오' as the last vowel regardless of how many syllables are in the stem (so, for example, prior to 1988, you'd have '아름다와' instead of '아름다워' for 'beautiful'). For polysyllabic stems, there was a change in the pronunciation over time and this change in the spelling reflects that. However, this hasn't happened (yet) for the monosyllabic stems.

The added 겠 change the meaning totally differently to the one that sounds sarcastic. So, it actually means, i dont think its good for u but it literaly means "its good for u" kinda ironic way to say.