Well, foliar feeding, which is what you would be doing with a sprayer, has its pros and cons:
It is more efficient at getting nutrients into the plant, without losses due to leaching, or binding to the soil. That is useful in poor, fast draining soil, or at extremes of acidity or alkalinity. Foliar feeding is also useful when you want quick results, often showing visible greening, growth, and/or bud formation within a week or two of application.
On the other hand, you have to be very careful about how you formulate the nutrient solution. Mix it too strong, and you can burn up your plants P.D.Q. Nutrients should be in a form that plants can use: Nitrogen should be in the form of nitrates, etc. Micronutrients, such as iron, manganese, zinc, etc. need to in the form of complexes, to help the plant's metabolism to process them, and to keep the phosphate ions in solution from binding with them. Also, with foliar feeding, applications need to be pretty frequent, since most plants have pretty minimal ability to store nutrients, relying on the soil to do that.

Depends on which plants and where they're growing - shrubs and perennials in the ground, foliar feeding is generally not necessary and there are plenty of granular, slow release formulations of fertilizer which can be raked or turned into the ground in spring. There's usually plenty of rain around to dissolve the feed in spring at least, and they generally don't need a second application, though you can if you like, according to the instructions on the product you use.

Plants in pots are even easier without using foliar feeding - something like Miracle Gro general purpose mixed in a watering can will sort those out, so I wouldn't personally recommend going the foliar feed route.

The sprayer you are showing is something I've owned, but it was only ever used to spot treat weed growth, especially in and around paving and paths. I'd also add that, if you're thinking of feeding now, don't - what plants are short of right now is not fertilizer, but water.

If you do go down the foliar feed route try to avoid spraying when hot sun is likely as the droplets can act as little magnifying glasses and make scorch marks on the leaves.
I'd only use it in an emergency or for plants on a steep bank otherwise hard to access (eg daffodils after flowering)

I would only use this for a bug infestation or as Tug says, a quick fix. Seems too laborious for routine foliar feeding which yields minimal benefits because most of it rolls off the leaves and gets taken up through the roots anyway...and to stand there with this 5 litre tank on your back is not fun. There's a better way! Work some organic fertilizer or slow release fertilizer into to soil around the root zone and give a good drenching - not this fine mist which blows away. It's got to penetrate down to the root zone. The plant will take up what it needs and can come back for more later.

Thank you so much everyone on your opinions, I knew it best to ask my gardening buddies.

I will not be struggling with my 5L sprayer on my back to foliar feed, LOL but will use slow release fertilizer which is much less work.

I will keep the sprayer for getting rid of bugs etc.

Thanks again, Poppy.