Many roleplaying games involve some degree of conflict which is resolved with lots of dice, multiple passes, earnest discussions with the Director aka the Dungeon Master, and comparing results to charts and character sheets. Some systems try to have a simple resolution which involves potentially a rock-paper-scissors chance task and a simple mental box where both opponents cannot run away (similar to Magic cards or related card games) to highly involved multiple chart reviewing and tape measures being extended.
Why use miniatures (minis) and a map with a printed grid versus using your imagination to set the battlefield? The answer is ultimately a style question and an audit system. Do you want to have an highly descriptive combat description with a quick resolution, then you want to avoid minis. Do you want to maintain a mystery and not give away the strengths and weakness of the opponent, then minis are not the answer.
Do you want to know where everyone is on the field and no magically teleporting players who happen to magically be where all the action is even though they have a movement rate that couldn't do it? Do you get tired of characters always shooting around the corner and hitting someone a mile away behind a large building? Do you have over 6 people involved in combat and you can't keep track? You are then a mini person.
How to do minis cheap? Poker chips, dry markers, and a wipeable plastic sheet. Let the players buy their perfect mini, its not your problem. Besides, it lets them be creative and paint themselves silly. If you have an artistic streak, then go forward paint those 100 goblins individually. Remember goblins are sentient beings so they all need to be painted uniquely.
Argument enders: mini tape measures. I use tape measures for LOS (Line-of-Sight) disputes and measuring distances so the power-gamers fireballs are fireballs not nuclear weapons.