So yes, full coverage metallic crowns are indicated for carious lesions as well as patients wherein the oral hygiene is poor.
However you should know whether it is for primary teeth or permanent teeth.
In permanent teeth usually full metallic crowns are given after the endodontic procedure. When the patient has extensive caries reaching pulp, endodontic treatment is done followed by a full crown in the posterior teeth. Usually in anterior teeth full metal crown is not required and they can simply be restored by composite/amalgam build up.
However there are certain indications for full metal crowns in anterior teeth also(1).
Full metal crowns in posterior permanent teeth is required since after an endodontic treatment tooth becomes brittle. Especially for posterior teeth wherein the masticatory forces may fracture the tooth full metal crown is beneficial(1).
A tooth colored crown, eg zirconia crown can also be given.
Now talking about the primary teeth. In primary teeth a non invasive method is preferred for a carious lesion which has not extended till pulp. This technique is known as Hall technique (2).
In this after confirming through a radiograph that sufficient dentin is present between the pulp and caries, then without removing the caries a full metal crown is placed on the tooth (2,3)
Now the question arises that what about the infection, it may progress?!
The full metal crown prevents further infection from the oral environment. The infection which is still there, yes it does progress but in a very slow manner. From a study (3) it takes atleast 5 years to progress at its full. By then the baby develops his/her permanent teeth.
Now in cases wherein the lesion has reached the pulp then depending on the case pulpotomy or pulpectomy is performed followed by the full metal crown coverage(4).
Talking about oral hygiene, in paediatric patients who cannot maintain their oral hygiene maybe because they are medically or physically compromised then a full metal crown provides a very good option to prevent the tooth from caries or plaque deposits.
Hope I have answered your question :)
It would have been better if you would have mentioned the extent of caries. Hence I have taken all the case scenarios.
(1)Are full cast crowns mandatory after endodontic treatment in posterior teeth?
Aseem Prakash Tikku, Anil Chandra, and Ramesh Bharti
(2)The Hall Technique; a randomized controlled clinical trial of a novel method of managing carious primary molars in general dental practice: acceptability of the technique and outcomes at 23 months
Nicola P Innes, Dafydd JP Evans & David R Stirrups
(3)Innes, N. P. T., Evans, D. J. P., & Stirrups, D. R. (2011). "Sealing caries in primary molars randomized control trial, 5-year results". Journal of Dental Research, 90(12), 1405-1410
(4)Preformed metal crowns for primary and
permanent molar teeth: review of the literature
Ros C. Randall, PhD, MPhil, BChD