Capital rarely has a single purpose. Some of it must stay reachable; some can be committed for longer in pursuit of a different objective.
Structuring deposits across time horizons is less about maximising a headline rate than about matching each portion of capital to when it is actually needed.
A useful starting question is not "what is the best rate available" but "what would happen if I needed this money back next month." Capital earmarked for a near-term need belongs in a shorter, more liquid tier even if a longer lock-up offers a better stated return — the cost of being forced to exit early usually outweighs the difference.
Splitting capital this way also makes a portfolio easier to hold through a downturn: the portion committed for the long term is not disturbed by short-term liquidity pressure elsewhere.