Well, as Robert says, tanks have over ATGM units the advantage of mobility, armor and more fire opportunities. SPMBT is very unforgiving with the firing rate of ATGM units, so they can be used nearly only in defense.
One other advantage of tanks over missile tank hunters: larger ammo allowance!
10-15 ATGMs are disposed of really quickly, particularly against mobile targets (not mentioning the 4 rounds of foot ATGM teams), while a tank can pack 30-40 AT rounds or more and make good use of them from a decent position.
So I tend to use tanks as the most versatile and resilient mobile platform around. Makes good use as long as you keep them in a balanced force mix and don't push your luck. Oh, and some kind of technological edge over the enemy's best (at least being able to punch through his best armor) can also help...
Re: why buy armor?
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