Learn Finnish: Tehdas tuottaa autoja sadoittain. - The factory produces cars by the hundreds.

The base word is sata. To add the distributive suffix -ittain, you use the plural stem of the word. The plural stem of sata is sadoi- because the consonant t weakens to d, and the plural marker i changes the final a to o. Adding -ttain gives us sadoittain, meaning 'by the hundreds'.

Could I just say 'satoja' instead of 'sadoittain' here?

You could say Tehdas tuottaa satoja autoja ('The factory produces hundreds of cars'), which is grammatically correct and very similar in meaning. However, sadoittain acts as an adverb describing exactly how the cars are produced ('by the hundreds' or 'in hundreds'). This places a much stronger emphasis on the continuous, massive rate of production.

Why does this word use the suffix -ittain instead of -ittäin?

This is a classic case of vowel harmony. The stem sadoi- contains the back vowel o (and the root word sata has a). Because of these back vowels, you must use the back-vowel version of the suffix, -ittain. If the root only contained front vowels (like ä or ö) or neutral vowels (i, e), you would use -ittäin, as in päivittäin (daily).