I take 3 grams of TMG (trimethylglycine or betaine) daily. I actually use the baseline series produced by @sns8778.
I take it for a variety of reasons.
First, for the reasons detailed by @Resolve10 in his post, i.e. athletic performance. And as @Jeremyk1 said for health purposes.
Second, I also take it as I take Nicotinamide mononucleotide (NMN), via the anti-aging protocol endorsed and formulated by Dr. David Sinclair. When NMN is used by cells to create NAD+, a molecule pivotal for health, nicotinamide is formed. This molecule needs to be disposed of. In order to do so, a methyl group is placed on nicotinamide so it can be secreted by the kidneys. TMG provides methyl groups (TMG consists of three methyl groups per TMG molecule) and fills this role for my purposes.
Thirdly, I've taken various forms of exogenous carnitine over the years and TMG not only helps the body produce carnitine, but supplementation also causes an increase in carnitine metabolites in muscle.
Relatedly, l-carnitine and betaine function as osmoprotectants to suppress inflammatory responses.
I've also just started taking injectable (IM & subq) l-carnitine @ 300 mg daily, along with subq MIC B12, and alongside the oral betaine dosing I've experienced a huge increase in athletic performance, vascularity and weight loss. I'm on day three of this protocol and I'm down literally 2.8 lb with no change to my diet, lifestyle or exercise frequency.