Ants do not do harm in the garden, as they are not vegetarians. They might even be beneficial by hunting some insects. However, they like nectars and sweet stuff. Sometimes saps coming out of an injured plan might attract them. Also, sometime some plants get this sticky thing on their stems/leaves .... ants like that too. They are not responsible if the plant dies or gets diseased.
But there is controversial thing that some say ants farm/ breed aphids(like a farmer breeds cow and pigs). Believe it or not !