The Poedit utility is not required to translate an application (a text editor can replace it).
However, to facilitate their work and to save time, I recommend to translators the use of Poedit (a recent version). In particular, I call your attention to the possibility of creating a database containing the translations already present in your system (this allows automating some of the new translations).
I prefer POedit version 1.4.6 in particular. For some reason, I can't use keyboard shortcuts in the most recent versions. For example, I can't use the TAB key to switch between the string list and the string edition box, and in the string list I can't use the arrow keys to move up and down. I'm trying to minimize my use of the mouse to avoid RSI (Repetitive Strain Injuries). I could use mouse keys to move the cursor, but that alone isn't enough, it isn't as quick as keyboard shortcuts.
You mean the spellchecker. Yes, you have to install the hunspell dictionary for your language (should be included in the langpack) and then edit the catalogue properties from POedit to set the locale of the file. Since I don't have some Portuguese characters in my keyboard, the spellchecker has been very useful.