I have seen many men wearing a waist band or thread around their waist, red or black color. It's called Karbuta in Marathi. Is there any significance behind wearing it?
You can not ask for scientific reasons here as it will be considered as off-topic. But you can ask for significance or just reasons behind some practice.
The waist band is prescribed in the scriptures in the dress code. It is called the "Kati Sutra". Without wearing this Kati Sutra one is not qualified to perform Srauta and Smarta rituals. That is why you'll find Brahmins or any other persons performing Puja or Homa, usually tying a piece of cloth or thread around the waist/loin.
Besides an upper covering be worn by all as krishnajina or the dried and treated skin of deer. Manu prescribed a ‘kati sutra’ or loin thread: Mounji trivrit samaashlaokshou karya viprasya mekhala,Kshatriyastu mourveejyaa vaishvasya shaanataantavi ie, three stringed mounji grass material to Brahmanas, twisted sring made of mourvi material and to Vaishyas a shaana made string. Samvarta stated that without a kati sutra, no male is qualified to perform Shrouta Smaarta Karyas! Manu Smriti instructs that without kati sutra, danda, yagnopavita, kamandala with water be secured as soon as possible!)
Bhrigu Maharshi disallows wearing deep red and black clothing, wet clothes and without borders. Kati sutram vinaa shroutam Smaartam Karma karoti yah, Sarvam tannishphalam vidyaatsopi nagna iti smritah/Sapta vaataahatam vastram shushkavat pratipaaditam, Shuchivohavyaa maruta ityuktvaa shuddamambaram/ Samprokshya Devasyatveti griheetvaa chaavadhutayet, Taraniru dutyamiti vaava vahanteeti chaadayet/
( No Punya Karma of ‘Shrouta- Smaarta’ nature be even performed without ‘Kati Sutra’ as all such deeds are invalid; so is a person desirous of performing such deeds without ‘kaccha’ or ‘uttareeya’ is as bad as nude and thus disqualified.
As you can see from the 2nd block quote, if a performer of Srauta or Smarta rituals is not wearing the Kati Sutra, then he should be considered as naked (nagna iti smrita) as per Rishi Bhrigu.
Both these quotes are from the book Dharma Bindu, an essence of Smritis (Dharma Shastras).
What you saw is probably a modern form of the mekhalā (girdle) which is tied to boys during upanayana (initiation). The Manusmṛti says the tying of the girdle is done as part of a saṃskāra called Mauñjībandhana:
baijikaṃ gārbhikaṃ cainaṃ dvijānāmapamṛjyate || 2.27 ||
Of twice-born men the taint of seed and womb is removed by the ‘Libations in connection with Pregnancy’ and by ‘Jātakarman’ (Rites attendant upon birth), ‘Chauḍa’ (Tonsure) and ‘Mauñjībandhana’ (Tying of the grass-girdle). (2.27)
Specifications of the girdle:
mauñjī trivṛt samā ślakṣṇā kāryā viprasya mekhalā |
kṣatriyasya tu maurvī jyā vaiśyasya śaṇatāntavī || 2.42 ||
For, the Brāhmaṇa the girdle should be threefold, of even thickness, soft and smooth, made of muñja grass; for the Kṣatriya it should be the bow-string made of murvā grass; and for the Vaiśya the cord made of hempen fibres.—(2.42)
muñjālābhe tu kartavyāḥ kuśāśmantakabalvajaiḥ |
trivṛtā granthinaikena tribhiḥ pañcabhireva vā || 2.43 ||
In the event of Muñja (and the rest) being not available, they should be made of Kuśa, Ashmantaka and Balvaja,—triplicated with one, three or four knots.—(2.43)
Wikipedia has a picture of a boy wearing a girdle (made of grass) around his waist:
A boy during his upanayana ritual. The thin, yellow Yajñopavīta thread runs from left shoulder to waist. Note the muñja grass girdle around the waist. The peepal tree twig in his right hand marks his entry into the Brahmacharya stage of life.
Besides being tied as part of a purificatory rite, I don't think the mekhalā has any other significance. You can read more about it in P. V. Kane's History of Dharmaśāstra, Vol II Part I.