Why is the number 2 needed for 2-methylpropanal?
Well I suppose you are correct in a way- but it does help us see the molecule quicker in our heads...

it should be written methyl-2-propan-1-al or methyl-2-propan-2-al
otherwise it would be a butan-1-al  or butan-2-al

let me explain further>
the new IUPAC nomenclature  advises the use of numbers instead of the iso epithet- in organic chemistryIn old naming -as with propanol and iso-propanol - so with propanal

Isopropyl alcohol

Preferred IUPAC name

Propan-2-ol

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Isopropyl alcohol (IUPAC name propan-2-ol; commonly called isopropanol) is a compound with the chemical formula C3H8O. -It is a structural isomer of 1-propanol.

wiki  Chemical Names: ISOBUTYRALDEHYDE; 2-Methylpropanal; 78-84-2; Isobutanal; Isobutylaldehyde; Propanal, 2-methyl- More... Molecular Formula: C4H8O or (CH3)2CHCHO.