Housemaids at max level give you a crucial buff that is 6% +1k boost to either your hp, matk or patk. It may not sound like much to some people but it's crucial to reaching endgame dps or hp. I think it takes over a month to get a housemaid to max level so don't wait until your level 70 to buy and level them, you will regret it.
It's important you buy the right types of housemaids for your class(es) because each housemaid can only reach a 100 skill level for one skill and you can only have up to 6 housemaids!
So if you need the patk buff, the housemaid you buy needs to able to reach a 100 skill level in the battle skill. For the matk buff it's the magic skill. And for the HP buff it's the defense skill.
NOTE: You can only use one buff at a time!! So if you're always a melee dps you probably wouldn't ever need a maid that gives you a defense buff or an matk buff. HOWEVER at max level the buff you get is actually 2 buffs, one for 6% and one for 1k. You can replace the 1k portion of the buff by asking for another buff and you will get like the crit rate buff along with your 6% buff. But you can't get two 6% buffs! If you get another maxed level buff with 6% it will overide your first 6% buff. If you had a housemaid with a non maxed defense skill for example, you could replace the 1k patk portion of your attack buff with some HP or defense.
You level up your maid's skills by using them but their skills can only go up if your maid has a higher affinity then the skill you want to increase. To level up your housemaid's affinity you must chat with your housemaid.
To keep it simple, you may want to chat up all your housemaids so they reach 90 or 100 affinity before you use any of their skills. That way you don't accidental use skills that are the same level as your maids affinity and not increase the skill level.
You can only use each maid 4 times before it gets tired. At the beginning, each time you chat them their affinity will increase by one point. But later on you'll have to chat them more for each affinity increase.
ALSO, some chats you do won't even count as a use!!! So you'll need to chat them a lot just to use up their 4 uses. You'll know you've used them when the maid says they're tired. You can also check their fatigue level to be sure but I never bother.
Please note, you'll be able to use or chat your housemaids again one time for each 5 hours or so that they rest, to a maximum of 4 uses.
Housemaids are also able to make very unique potions and food for free. Most of the food they make only lasts 5 minutes but it's free! For melee classes, it's strongly recommended that you have one cooking housemaid and limit it's cooking skill to lvl 40 by keeping his/her affinity at level 40.
This way you always get lots of caviar sandwhichs, which will give you 10% extra crit damage for 60 seconds, a must for boss fights.
What types of housemaids you hire is totally up to you. However, maids don't always give you the crucial 6%+1k buff and may give you the less useful crit buff 4 times in a row!! So having 2 maids for each type of buff you may need is recommended.
Keep in mind that you don't really need a cooking or pot making maid with a 100 skill level because all your maids will still be able to get up to level 80 in all skills. HOWEVER, you probably only need 2 of the 3 possible buffs at most, so getting 2 maids for each buff, 1 for cooking and 1 for pot making, MIGHT work good for you. But, just for example, you could opt to have 3 for your attack buffing, 2 for your hp buffing, and one for pot making since you may use codex food anyways. REMEMBER, you can use the maids you're not using to buff to make more potions for you!
Another important thing to know is that charisma determines how long the buff you get lasts and how many potions/food you make per use.
So when you buy your maids, try to get ones with 100 in charisma. Housemaids sell out sometimes or the good ones get snatched up quickly so keep checking what's available until you get the ones you need. I think they reset every day with daily resets so be quick!
If you're maids are rude to you kick them in the teeth and show them who's the boss!