You ward a area, by scuttle lets say. Grag walks over it, that helps you telling you that he is in the area, it helps bot allowing them to push up and play aggressive. it helps the jungler to choose whether he can counter gank, gank mid/bot, counter jungle, or solo dragon. Wards help everyone on your team, not just you.
Pinks aren't expensive I generally always have enough for a pink, As a mid laner I try to 1st back with 1,075 so I can get a Lost Chapter, 2 HP pots, and a Control Ward. If I need to back before then I try to always have a extra 175 (2 HP pots and a control ward) so if I back at 475-550 I get boots plus pots/wards if I back at 575-850, Another Dring Pots/Ward, 875-1959 Boots, Dring, Pots, Ward, I make a effort to keep shit warded for myself.
IF you pink the river bush it also allows your Jungler to know its not warded right there for a impending gank.