Circle spells require spell slots
First lets start by looking at the rule for Circle Spells (emphasis mine):
Your mystical connection to the land infuses you with the ability to cast certain spells. At 3rd, 5th, 7th, and 9th level you gain access to circle spells connected to the land where you became a druid. [...]
Once you gain access to a circle spell, you always have it prepared, and it doesn’t count against the number of spells you can prepare each day. If you gain access to a spell that doesn’t appear on the druid spell list, the spell is nonetheless a druid spell for you.
Nothing in this ability modifies how you cast spells, only granting a list of prepared spells that still use spell slots. However, it has a few good benefits you may have overlooked.
Additional prepared spells increases flexibility
By default druid's prepare spells according to the following rule:
You prepare the list of druid spells that are available for you to cast, choosing from the druid spell list. When you do so, choose a number of druid spells equal to your Wisdom modifier + your druid level (minimum of one spell).
Particularly at early levels this small number of options can be quite limiting. Do you take that extra healing spell or an offensive one? By giving additional spells that do not count against this limit Circle Spells increases your flexibility and the options available to you when preparing spells.
Access to powerful spells not on the druid list
I've talking in another answer about how the druid spell list isn't really that powerful. By default they have less powerful offensive options available than wizard/sorcerers, weaker buffs than cleric and less manipulation than bards. Their spell list lets them do a bit of everything without being great at anything.
Circle Spells help address this by giving access to some great spells that are otherwise unavailable to druids. Even better, these spells count as druid spells for you, so you can use wisdom as your spellcasting ability.
Why not choose a different circle?
Circle of the Land is for druids who want to be pure spellcasters. They have access to more and more powerful magic than the other circle and don't have to waste their time turning into fluffy critters (circle of the moon) to be dangerous. In addition to the benefits above they have a couple of features that make them better spellcasters than other druids.
The bonus cantrip is nice, it means you don't have to choose between a ranged damage, melee damage and thematic cantrips, you can have them all! Unfortunately druid cantrips are fairly weak in general compared to other classes, but this is still better (for spellcasting) than anything other druids get.
The bigger benefit is Natural Recovery:
Starting at 2nd level, you can regain some of your magical energy by sitting in meditation and communing with nature. During a short rest, you choose expended spell slots to recover. The spell slots can have a combined level that is equal to or less than half your druid level (rounded up), and none of the slots can be 6th level or higher. You can’t use this feature again until you finish a long rest.
This feature gives a big boost to the number of spells you can cast per day. When combined with the Circle Spells this question is about it is the main reason Circle of the Land is the go to for pure spellcasting druids.