Every God (or God family) is capable of doing all things for the devotee. This is inpsite of being closely associated with a particular trait (Eg. bRhaspati as the guru, indra as warrior, savitR as the impeller).
Just because a doctor heals well, it does not mean that he disappears as soon as he removes his coat. He can fight and engineer as well.
In a sense, the basic essence of other Gods is present in every God - there is just a difference in emphasis. So, in inspired hymns, God X may say: “I am Agni, I am Indra etc..” When Agni is “angry”, it is as if the rudra aspect shines forth through him - or as if rudra fully cooperates with him, lending his features (in line with the harmony of the Gods).