At the present time, the relationship between a gallery and an artist is legally defined as a prinicipal-agent relationship. Which can also colloquially be seen in the idea that an artist is represented by a gallery. This can definitely be true in an ideal situation and in long-term relationships between artists and galleries, where it can truly be said that a gallery represents an artists' best interest. The perks of this kind of relationship, such as an exclusive right to a provision on any sale made within a certain market and which are otherwise legally questionable due to competition laws, are then justified, as the gallery has put in a considerable amount of effort to attain to the artists needs, whatever they may be.
It happens quite often, however, that artists agree to a relationship with all these caveats while only having done one exhibition together, or sometimes none at all. They are thus unable to know if the aims of the gallery are the same as the aims of the artist, as this can only become clear over time after the gallery has had a chance to engage the public about the artists work and actually perform its role as mediator between the artist and the public. It is therefore also not uncommon that the relationship between an artist and a gallery doesn't meet the expectations of either party and grinds to a halt within the first few years of collaboration. This of course is unwanted for both parties. Galleries made an investment they will likely get little to no return on, while the artists have been hampered in their freedom to pursue meaningful possibilities elsewhere. I believe it would therefore be advisable to any first contract between a gallery and an artist to have two stipulations.
The first is that the agreement should be entered into for a set period of time and that this period does not exceed three years. This gives both parties an incentive to make the relationship work, as the relationship needs to be renewed with both parties explicit consent.
The second is that the gallery does not retain a right of exclusivity for the duration of that contract. This means that while a gallery gets a provision on any sale made that it is directly involved in, it doesn't for any sale made made by the artists themselves or through a different gallery. This ensures that the relationship between the gallery and the artists will be mutually beneficial, as the gallery is forced to show the artist that its exhibition venue is the best place for sales, that it can bring clients in, pay its bills on time and do other things a good agent should do. At the same time, it provides artists with some amount of leverage over the gallery, something which they otherwise simply do not posses. If the gallery doesn't do its job well, artists can always simply exhibit at a different gallery or choose to do sales by approaching potential buyers themselves.
If a gallery however does a good job and the artist is happy with their relationship, it makes sense to enter into a full principal-agent relationship, whereby it can be said that a single gallery truly represents the artists and is able to speak for them. The legal securities such an agreement entails, which befall largely to the gallery, are then justified.
This might relieve some difficulty in younger artists, who are often so grateful that a gallery is willing to sell their work that they place themselves in a wholly dependent position, without any security that the gallery will value their interests over those of other artists or a longer period of time.
At the same time, it might relieve the pressure on the galleries to 'scoop up' artists as early as possible, often before their work and practice is quite up to the challenge, before finding out they don't work well together, dropping them and effectively stunting the artists' careers while also wasting their own resources. A more careful approach with greater returns over a longer amount of time is facilitated in this manner.
As a final note it can be said that this arrangement leaves most of the power in the artists' hands during the first years of a relationship with a gallery, which perhaps seems unfair to the galleries. But as a gallery, if an artist wishes to sell their works elsewhere and instead of enticing them otherwise, you contractually have to forbid them from doing so, you can't honestly speak of representing their best interests in the first place.