Is the clock buffer needed for every clock generated by PLL?

Arguably, there is no need for it (depending on your requirements). You certainly don't need one for an design to work.

As for what the ALTCLKCTRL core is for (straight from the user guide (https://www.altera.com/en_us/pdfs/literature/ug/ug_altclock.pdf)):

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The common applications of using this megafunction are as follows:

■ Dynamic clock source selection

■ Dynamic power-down of a clock network

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Based on what you've written, I wouldn't expect you to need it.

Cheers,

Alex